Now this is interesting:
"... while this rifle appears to be in some obscure and short 9.3mm cartridge"

In the cartridge collection that I inherited from my Dad is this - on the right:

9,3mm bullet (appears to be the 12.5 g or 193 grain flatpoint bullet for the 9,3x72R, see the post-war RWS bullet for comparison

Case length is 38 mm

Head dimensions similar to 6,5x54 Mannlicher-Schönauer

Headstamp reads: R.W.S. N
(generic RWS before WW II)

fuhrmann