When I got my first caliber .375 numerous years back there was no brass to be found. So I cut down some 38-55 and it worked fine. I think they were Winchester... but this is some years ago.
I did not do this for extra case capacity. You are not going to gain that much room. Plus I do not load my lever flippers to Max pressures. If I need that extra zip I will get out a bigger caliber.
I will likely get flamed for this but I have used 38-55 cases cut to 375 length and found that the 38-55 cases weighed 15 grains less than 375's and also hold 3.5 grains more water. I have not split the cases cases open to compare but the logical explanation seems to be that the 38-55 cases must be thinner than 375's to give this difference in weight and capacity. If that is the case , will the case take the increased pressure of the 375 ?
I'm not a pro on this subject at all. It seems as if most people who are actively shooting this cartridge are using lead bullets. With the scarcity of jacketed projectiles that's what I'm doing. My loads end up not being much more if any than a good stout 38-55 load anyway. It's more fun this way for me anyway.
I did try some short 38-55 brass loads in my 375 the last time out, with poor results. May try other loads in this brass soon.
The 375 chamber is .008 larger than the 38-55 chamber so the 38-55 should drop right in. The long ( 2.125 ) Starline brass is too long for the 375 and will extend into the throat of the 375 even though it will drop in. Shooting the long brass in the 375 is not a good practice and should be avoided.