Still looking for positive proof.

Warren, seems I was right and wrong.

You will find most Germanic guns triple serial numbered, barrel/frame or receiver/slide or bolt, due to law going back to proof tradition.

Glock continues to do so, even on guns made in the USA, in anticipation of any laws on a global scale. This latter statement a paraphrase of what Sweeney was told by Glock reps for his book.

Good information, I hope if more information,please contribute. Thank you.

All I can add anecdotally is that the three SN thing extremely common in Europe, especially Austria and Germany, from old Steyr Hahn all the way up to modern SIG on guns assembled there. A more entertaining past time might be had by trying to find a German or Austrian gun which does NOT have SNs on the three major components.

During WWII production of M1 carbines and M1 rifles ("Garands") (and M1911A1s?), manufacturers were forced to submit components for cross-manufacturer fitting. This was a Pretty Big Deal for the time and caused some consternation for some manufacturers.

Before that, firearms were largely hand-fitted and parts were hand-tuned to work together. Machining tolerances were not quite good enough to mass-produce parts for tinker-toy builds like we enjoy today with AR platform. As such, once a functional firearm is complete, the S/N (or parts of) is stamped on core components that are likely to be finicky if swapped.

Two of the popular-yet-notorious platforms to "rebuild" with fresh parts are the P08 Luger (just about everything) and the M1911 (slide, barrel).

Yep, and understood (by most folk trying to fit a barrel), and even though hand fitting required, not numbered stateside on 1911s. Buy a German assembled 1911 and it will have all three parts numbered. Glocks done that way even though likely a drop-in fit.

My guess is that some traditions are not easily abandoned in "old country" locales.

I still think the traditional is written, as in proof laws, for the above mentioned integrity, even if perhaps outdated on some guns. Even in the USA, SNs may not be stamped, or even proof marks, but quite a few guns have assembly marks keeping key components together, and only difference being marks chosen, whether an internal to company number, or simply using the number required still by many countries, the SN. Glock plainly said this latter part, due to laws worldwide, and being in compliance thereof.

According to Glock Deconstructed by Patrick Sweeney, page 83, Glock serial numbers the slide, frame, and barrel of a pistol to comply globally with all known and anticipated laws.

There ya go, you answered Warren's question straight up.

Ah, and I kept glossing over the important phrase you repeat in your posts -- "proof law".