This does not answer the original question but, prior to the 375 JDJ, Ken Waters 'rescued' an old Mauser, if memory serves. Had it rebored to .375 and made cartridges from the .444 Marlin. He called it a .375 Express. Dimensionally, there is only a couple of thousandths of an inch difference here and there between it and the JDJ.

I have a rifle which is marked as '.375 Express' made from a P-14. The original owner decided to use 400/350 brass for reasons unknown. The rifle came with 2 sets of dies and 100 pieces of formed brass. The only difference I have found is that the 400/350 rims are thinner than the .444. I use JDJ reloading data for it. One of these days I will try forming some of the Marlin cases and see if the will feed and fit.