Welcome to NE!
Interesting read at this link, certainly seems a possibility that your rifle in question could have had the (very) minor alteration to the chamber as well.
A chamber cast, or, like the article below, a fired case would tell the tale. You could get a rough guess by just (gently) trying to chamber a new unprimed 8x57 case. If the bolt closes then suspect it has been altered to 8x57 and proceed accordingly. .

Be aware that the Mannlicher’s feed from the magazine can be very sensitive to cartridge overall length as well as bullet profile. You will likely be limited in your selection of bullet weight and need round noses.

With a 1mm difference in case length I would trim after sizing.
- Mike

https://www.rifleshootermag.com/editorial/mystery-mannlicher/83682