If it is a .358" groove dia. then it would be a 400/350 which shares the same parent case with the 400/360.
I very much doubt that it will be a 350 though as that was a Rigby specialty and 360s were a common W.R. chambering.

A .358" cast bullet is perfect for paper patching to .366" though. Maybe that is what the previous owner used that mold for.