Hallmarks

The letters in a United Kingdom hallmark name the party who submitted the item

A set of initials inside a small shield is sometimes called a maker's mark informally. The United Kingdom assay offices define it in different terms: the mark identifies a registered person or company under whose responsibility items are submitted for hallmarking.

The definition names a registered party, not a maker

Assay Office London defines the first stamp on a United Kingdom hallmarked piece in one sentence: "The sponsor's mark identifies a person or company registered with an assay office so that items can be submitted for hallmarking under their responsibility." The same source notes that the mark is sometimes informally called a maker's mark, and defines it in the terms above rather than in terms of manufacture.

The Goldsmiths' Company states the same thing from the reader's side: "The first mark is the Sponsor's or Maker's Mark. It tells you who sent an item in to be assayed." Sending an item for assay and making it are two separate acts. The definition covers the first, and says nothing that would make the two the same party.

That is the whole of the answer to the question this page is about. Everything below is the form of the mark, and the readings the definition does not support.

What the definitions state about the mark's form

Three United Kingdom sources describe the mark: Assay Office London, the Edinburgh Assay Office and the Goldsmiths' Company. They agree on its composition and on its place in the sequence.

At least two letters, within a shield.: The Edinburgh Assay Office states: "In the UK, this mark consists of at least two letters within a shield." Two is the floor the sentence gives; no maximum is stated in this record.
The initials are chosen, and the shield is part of the mark.: Assay Office London: "The Sponsor's Mark comprises of the initials chosen by that person or company inside a surrounding shield shape." The surrounding shape is described as part of the composition, not as something separate from it.
It comes first in the sequence.: The Goldsmiths' Company names the sponsor's or maker's mark as the first mark of a full traditional United Kingdom hallmark; the remaining components, their status and what each of them reports are set out on a separate page here and are not repeated.
The form statement is scoped to the United Kingdom in its own wording.: The Edinburgh sentence begins "In the UK". It is a description of a United Kingdom mark by a United Kingdom office, and it is not a general statement about initials stamped on jewelry anywhere else.

What a set of initials does not establish

One reading is closed off by the definition itself. Because the mark identifies a registered party under whose responsibility items are submitted, the letters cannot be taken as naming whoever worked the metal. Where those are different parties, what the stamp records is the registered one.

The rest is a matter of what the sources contain. They define the mark and describe its composition. They do not attach any particular set of letters to any particular person or company, so no attribution is made on this page and none can be derived from it.

They also say nothing about age, origin or worth. A sponsor's mark is not presented here as evidence of when a piece was made, where it came from, or what it is worth — not because those readings are wrong in principle, but because the records used here do not address them at all.

Nor do they describe how a counterfeit mark could be recognized. That question is outside everything read for this page.

Recording a mark in front of you

These steps describe the mark. None of them attributes it, and none of them requires a source this page does not have.

Count the letters. The United Kingdom description is at least two letters within a shield.
Record the surrounding shield shape together with the letters; the London definition treats both as parts of the mark.
Note which stamp in the sequence you are reading. The sponsor's mark is described as the first.
Keep the letters as letters in your notes. Turning them into a name needs a source of a different kind, and no such source is used here.
Write down which country you are treating the piece as belonging to, since the definitions used here are United Kingdom definitions only.
Keep the sponsor's mark separate from the fineness numerals in your notes; what a figure such as 925 or 750 reports is a different question with its own page.
If you photograph the stamp, keep the original photograph as your record of it; the reading is of the object.

What this record does not cover

It names nobody. No sponsor, firm or individual is identified, matched or guessed at from a set of initials.
It does not date, place or value a piece. The definitions read here say nothing about age, provenance or worth.
It is a statement about the United Kingdom only. Other countries' marking regimes do not appear in these sources, in either direction.
It does not cover the other component marks of a full hallmark or which of them are compulsory, and it does not cover what a fineness numeral reports; both are handled on their own pages.
It does not cover age exemptions, laser marking against struck marking, fees or enforcement. None of that appears in the pages read for this record.
It makes no claim that the app reads, identifies or preserves a sponsor's mark in an image.

Questions

Do the letters stamped on a piece name the person who made it?

Not by definition. Assay Office London describes the sponsor's mark as identifying a person or company registered with an assay office so that items can be submitted for hallmarking under their responsibility, and the Goldsmiths' Company describes the same mark as telling you who sent an item in to be assayed. The party responsible for submitting an item need not be the party that made it, so the letters answer the submission question rather than the making one.

How many letters does a sponsor's mark have?

The Edinburgh Assay Office states that in the UK the mark consists of at least two letters within a shield. That is a floor rather than a fixed count, and no maximum appears in the sources read here.

Is "maker's mark" simply the wrong term?

Both names appear in these United Kingdom sources. The Goldsmiths' Company writes "the Sponsor's or Maker's Mark", and Assay Office London records that the mark is sometimes informally called that while defining it as identifying the registered party responsible for submission. The definition does not change with the label used for it.

Can I use the initials to work out who made a second-hand piece?

Not from anything on this page. The sources here define what the mark is registered for and how it is composed; they do not connect any specific set of initials to any specific party, and no such connection is made or implied here.