No Difference? Just count the number of sidelock pins on the 304 pictured above and compare it to photos of any 303...the 304 has two sear trains on each side (safety interceptors)...as far as I know, all post war 303s have one sear train only (each side)...and...I think all 304s are pre WWII...note no hand detachable locks like many 303s have and the protruding barrel lump indicates a quad lock-up (just like most 303s)