Not like the .375 Flanged brass is any easier to source today than the 369 Purdey anyway.
375 Fl Brass isn't hard to find.
I believe a fair few of the purdey's were rechambered around the same time the Rigby's (the 350's that were made into 9.3x74R's (and my 375 Fl) were by Paul Roberts.
A fair amount of this type of thing seemed to go on looking at what comes on the market.