The Astra 900 was a pistol developed to take advantage of a large Chinese demand for semiauto pistols with shoulder stocks, following on the massive sales of the Mauser C96 "Broomhandle" in that country. In the 1920s and 30s, civil war in China drove a huge market in arms, but international treaty had prohibited the importation of military arms. One loophole in the accords, however, was that stocked pistols (including fully automatic stocked pistols) were not considered military arms.
Mauser led the sales to China, but several Spanish companies jumped into the market as well, and the Astra 900 was a well made and reasonably popular gun.The semiauto 900 was chambered for 7.63mm Mauser (like the C96), and about 21,000 were made from 1928 into the 1930s. A series of improved designs followed, with the 901 (select-fire), 902 (select-fire with a fixed 20-round magazine), 903 (select-fire with detachable magazines), and 904/Model F (select-fire with detachable magazines and a rate-reducing mechanism).
Mauser C96 with 20-round magazine: https://www.full30.com/video/40c45d37d368ebe195736653d694e1d3
Scoped C96 Sporting Carbine: https://www.full30.com/video/ff3995dc5d05724a2da89fc3e9a06873
Mauser 712 Schnellfeuer: https://www.full30.com/video/0795387128863461640faffd4bb8a3ad
Shansei .45ACP Broomhandle Mauser: https://www.full30.com/video/16fa17b1475706b987571591c11c81c3
Luger 1902 Carbine: https://www.full30.com/video/972a370125b4407647e94601ca4699cf
ATF C&R List, including NFA exempt guns: https://www.atf.gov/files/publications/firearms/curios-relics/p-5300-11-firearms-curios-or-relics-list.pdf