Finally came back around to this thread...
Max, the 461 #1 and 461 #2 have the same dimensions except in neck length, the #2 being longer.
So, the #1 case will chamber in either rifle.
The .465 snap cap you have is probably a 500/465 H&H (nitro express) so is quite different from the Gibbs.

Do you have any updates or additional pictures of the rifle?