Quote: "I believe the bottom metal may be siamese to accept the short rimmed 375."
Imho Fraser bought in some (know of 2 others)original Mauser "short intermediate" actions through Rigby. Rigby used these rare actions with slant magazines for .303 Mauser M98 rifles. AFAIK only about 50 such actions were ever supplied to Rigby by the Mauser, Oberndorf factory 1898 to 1908.