The .303- 375 was made years ago primarily for Northern territory Buffalo hunters. It was called the .375 Territorian. it was a solution to the Buff hunters need for more power. At the time .375x2.5 brass, dies etc were simply not available , hence the Solution was the Territorian. just resize .303 brass.
Nowadays it is easy to access dies, cases and have a barrel reamed in .375 x 2.5 and loading data is also available.
I built one on a No 3 a few years ago using a TSI barrel. a local gunsmith had a reamer and hey presto, . I still have the Singe stack magazine that was very reliable. of course I regret selling it so another is in the wind.

A friend of mine has built a couple of interesting rifles on no 4's. he uses 8x56R Steyr cases, .373H&H slightly modified dies and uses the H&H reamer. Cases are readily available, its easy to chamber a barrel, and it puts out considerable more power than either .375X2.5 or the 'Territorian"
He's trying to convince me to go that way rather than another .375x2.5. A very convincing argument. Just a thought. Hendo.