At least according to NRA,using 375Win in 38/55=bad idea:too thick.
Scuttlebutt has it, that you can use 30/30 brass[necked upto.375] with projectile seated out[to make up shorter case length]in a 38/55.30/30 came from 38/55[parent case].
Supposedly Starline is planning to come out with 38/55 brass.
If I had my druthers,I wish the Contender had been in 38/55,rather than 375Win.