In the 1920s and 1930s there was probably little market for surplus sub-caliber devices. Parts and labor were cheap and available.
There was a market for low cost Krag rifles adapted for Hunting. Bannerman, Stokes Kirk, and Sedgley are well known for cobbling together salvaged parts to make improvised, sellable guns.

Worth noting:
Krag receiver rings often seem to have other symbols or letters stamped on the front face, in addition to the proof "P" and barrel index-mark. I assume these are inspection stamps.

It is interesting that Mark's receiver has a "W" at the 6 o'clock position. This begs the question if this "W" might have denoted a receiver earmarked for use at Watervliet Arsenal?

'FredC' - As always, you bring up interesting points.

I have been thinking about and trying to understand the process, sequence, and rationale of Springfield stamps and marks for years. (British, German, and French ones, too)!

The marks mean something. They were done with a purpose and communicated information to someone at sometime.

I once assumed gun makers stamped metal components when they were 'soft' and before they were heat-treated and finished.

However, final inspection and proof stamps make little sense if they are done before a component is Finished and Correct.

Now, I don't know for sure and freely admit my puzzlement.

Some U.S. Krag markings look like they were done in the back room of a 'Khyber Pass' gun-shop. There are lots of funny hieroglyphics around the magazine opening and on the back of the bolt-handle root, odd 'steel lot' symbols, periodic changes in serial number size, style, and font. Some numbers and letters are very difficult to read 'correctly'. Some stamps appear very primitive given that they were done in a relatively modern 'machine age'. The system appears like it was always changing. Such is the charm of Krags!

I agree with Dick Hosmer. The OP's Krag did not leave Springfield Armory as it is now assembled. SA would never have compromised a Gun Stock by gouging out all that extra wood. It is not a model 1898 carbine. The most interesting aspect of this Krag is the novelty of the receiver!

We have not seen a close picture of the front-sight. So I am not sure if it is a carbine barrel. (This is actually irrelevant. All types of Krag parts were in the hands of surplus dealers). This is a 'parts gun' assembled outside SA.

One thing that impresses me about SA 'markings' is that they don't seem to disrupt or displace the surrounding metal. Oddly, sometimes a SA marking will be inconsistent in its depth. Clarity of letters and numbers is variable.

IMO-Fake marks are noticeable because they disrupt surrounding metal and finish.

He are some more numbers that is on front of receiver. Nothing stamped on the gun has any raised material around any stampings.

IMHO - "P" and other letters on front of receiver ring are clean and free of deforming. Sometimes the "P" is not deep or clear.

Somehow, SA seems to have cleaned up around stamped numbers and letters, even if the steel was hardened??? It makes me ponder.

I posted a picture of a receiver that shows SPRINGFIELD heavily struck at both ends, but, lightly struck in the middle. All I can envision is that the Die had begun to bend?

Here is a peek at "P" mark of two more model 1898 Krag receivers.

1st two pics are of a '98 receiver made around November, 1899. The "P" is almost impossible to identify as a P. There are various scratches from stone or file work on receiver face. A lot of the case-color survives.

The last two pics are a '98 receiver made around April, 1903. Markings are clear, but, the finish is probably Armory refurbish prior to WW1. Also has a "DD" stamped on front face of receiver.

Proofing was one of the first things done, before ANY finishing operations. Note that the last "proofmark" (actually more of an inspection for functioning, since the barrel was deemed good at that time) was the (P) which was struck in the wood - no worries about buffing over. Yes, the receiver and barrel had different finishes - were they disassembled after once being joined? I was not there, but I'd have to say "NO". You can get a fairly decent picture of Krag fabrication from reading OM22. VERY little changed between 1878 and 1892.

It may be that there was an intervening process between the TD and the '03 which called for a proof cartridge AFTER assembly - just don't know.

Distinction needs to be made between "proofing" and "guaging". While they needed to weed out any bad apples, they HAD to have pretty well satisified themselves that the barrel was sound before expending a lot of labor on it. No one expected a rifle to fail structurally after assembly.

As to timed threads, how else would you do it? The extractor cut has to line up - it isn't made later after they thought the barrel was 'tight enough'. They were clearly able to index the threading and machining of the parts such as that when the witness marks aligned, all was good.

The metal has to be displaced whatever speed it is done at. I would guess that a fixture was used for the US SA stamp with a hand lever toggle set up to get enough pressure. I have used and built air toggle presses and you probably use hand operated toggle press to reload. The serial number could have had a stamp holder with replaceable elements or rotating wheels. It may have been semi automatic as it would be difficult to number receivers all day long without using the same number multiple times and skipping others. When you have done production machining for 40+ years it is easy to visualize all the things that can and do go wrong.