The .375's are standard length actions opened up to take the .375, or at least they used to be.

Many years ago I corresponded with Jack Lott. I was moving to RSA at the time and he offered to make up a .458 Lott for me. IIRC, his method was to use a .458 Win, ream to .458 Lott, open the action rails and replace the bottom metal with .375 H/H Mag metal and he also inletted and set in bedding epoxy as part of the rebedding of the action. He offered to do the whole job for $300. He considered the Zastava actions to be good working-gun actions. Remember, this was c.1985 and fewer CRF actions were available then.

I really had no use for a .458 Lott at the time, was pressed for time and...didn't have the spare $300 {!!} and never had the gun built.

Yeah, I know, one of those "What was I thinking???" moments...