ooops EDITED:
My comment was based on “possibly” .... ie: “could maybe be”......and the author’s experience in the linked article.
In other words, best to check to be sure.

Here in the USA it was (seemingly more prevalent post WWII) somewhat common for rifles in less available chamberings to be altered to more available ammunition.
Would it not be prudent for the OP to check out this “possibility”?

This happened to not only surplus military rifles, but sporting arms brought back after the war.
Remember, here in the USA there are NO mandatory or government imposed rules for reproof or marking of chambering on either new or altered ams. Buyer must beware.
- Mike