The proofmarks shwn tell a lot more about the history of this M 1900 Mannlicher – Schoenauer, seial number 295: The rifle was first proofed in Austria in 1901. Quite soon it came to Germany. As the Austrian proof was not valid in Germany pre-WW1, it was proofed again as a 6.5x54 before 1911, while the old gauge numbers were still in use. The CROWN – crown/N proofmarks indicate proof using the "4000 atm special rifle proof powder", while the gauge number 278 stands for a bore/land diameter of the barrel of 6.5 mm. Much later it was rebored/rerifled and rechambered in Germany to an 8mm cartridge. The "Repair" crown/R mark is faintly visible abve 7,8 mm. It was reproofed 935 = September 1935 by the Zella – Mehlis proofhouse, shown by the ledger number 16. But you should recheck the "new" chambering: The 7.8 mm bore / 57 mm case length speak more for a 8x57 I. The minimum bore diameter of the 8x56 M-S was 7.9 mm then. In 1935 the 8x57I was a common hunting cartridge in Germany, while the 8x56 M-S was a rare bird.
Btw, this M1900 serial number 295 in "8x56" was listed by another Mannlicher collector before with the remark "Poulin Oct. 2017".
You are asking for a "spring pin" for the foreend of your rifle? Spring pins were peculiar to the Take down Mannlicher-Schoenauers . Common on all other Mannlicher – Schoenauers were those stirrup type sling swivels with a screw like on the M1900 number 198 on GunAuction.com. That rifle was reproofed in Ferlach 242 = February 1942 btw. Stirrup sling swivels are available from Brownells and others.