The Mausers that Rigby’s 400/350 was built upon was an entirely new action from Mauser, not a modified standard action.
All Rigby did was tell Mauser what they wanted and Mauser made it.
Also, there was some amount of hand loading going on in the early 20th C.
Certainly not to the extent that it was done in the BPE era but Kynoch did provide cases, primed and unprimed, as well as projectiles as did Westley Richards with their proprietary LT capped bullets.