Not quite old or obscure enough for our beloved Forgotten Weapons, the M9 handgun is still in service with the US Military, though the Army now has the Modular Handgun System RFP bids in (but will take at least a year to test and determine the winner).
With its modern service record, for better or worse, few have truly dug into the full history of its creation, specifically its locking block. Those looking at the handgun typically only note the open barrel design, which is highly unusual for handguns.
WeaponsMan has a great article up on the history of the pivoting block design. Originally patented by Beretta engineers in the 1950s, it was the culmination of a series of inspirations including the Walther P-38. In the article, WeaponsMan traces the roots of the base M9’s design all the way back to the 19th century, citing the various patents (and showing them, where possible).
The first inspiration was from the Feederle brothers who were the original developers of the Mauser 1896 pistol commonly known as “The Broomhandle”. From there, the brothers improved upon their base design and introduced a pivoting locking block in 1907, using short-recoil operation.