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Great rifle!
I have read (some right here on NE) that the rimless Fraser Velox really is the 7.65 Argentine loaded with Fraser's special ratchet bullet.
There is also a rimmed version apparently using a 303Brit case and the Fraser bullet.
I am thinking your rifle could be chambered in either but more than likely the rimless.

Do a chamber cast (which is a breeze on a detachable barrel) and see if there is any difference to the Argentine case. If there is, it will probably be in the neck and shoulder which can be easily fireformed.
Do a search here. WVFred has or had one of these.