I just bought this MS carbine. It is in great original condition with a Voitlander scope and was advertised as a 1903 6.5x54MS. It looks like it is a 1903. However, the only marking visible on the top ring is the "Patent" due to the scope base dovetailed in. The weirdest part is the caliber marking. It appears to be marked as "6.4mm53-1/2". What the heck is that?

All numbers match and it doesn't have many appreciable proofs on the underside of the barrel like others I have seen online.

I planned to get some 6.5x54MS ammo and see if it chambers correctly but thought I would ask the expertise here as well. I can not find a "6.4x53-1/2" mentioned anywhere when I Google it. Do you guys know what this is?

Thank you.