Since acquiring an original 6.5x58 p.m. which I reform brass and handload for,i have done quite a bit of research.....mine is one of the earliest ones made.....no small locking screw on bottom of floor plate and all matching serial numbers...mine is 7112 ,no letter prefex and in very good condition with pristine bore..
1) ALL original 6.5x58mm have the front receiver notched to accomidate the loooooooong 160 grain round nose projectile (Cruise missile type like the 6,5 x52 carcano). note that the conversion round 8mm is actually SHORTER than the 6,5x58mm.
2)in 1905 or 06, there was actually a factory model 1904CARABINA with a short barrel and chambered in 7 x 57 mm. not a conversion but an actual ,separate model made for the nationaléregional police in BRAZIL. ( Could explain the BRAZILIN Bayonet..)
you might try a 7.57 mauser round.....even just the fired case.......remove bolt, hook the round into the bolt, re insert the bolt, carefull not to knock the case out when reinserting.... Might need a toothpick to insert case ......... these rifles were made in BERLIN with exhisting available parts...... could explain the 6.5 on receiver ...that being said, its hard to imagine a DWM inspector letting a 7x57mm leave with the old 6.5 on the barrel.....again the 7x57 is NOT to be confused with the 8mm conversion...
as to the value, well,i am so pleased with mine, I would not sell it for any reasonable amount...........cheers POPS