saxon & geert45,
First, My name is Burgess, NOT Bob.
Frame numbers: Found on early .25 caliber Mauser pocket models. Located under the magazine catch spring. On the earliest examples the number is the same as the serial number. At some point the numbers become out of sync with the serial number. This may of occured during the WWI era when production of the .25 caliber model decreased dramaticly and may have even stopped for a period. There is also a gap in the serial number range on or about the same time. The frame number may of been used as an inventory number at some point. Also, at some point the frame numbering stopped. Initial research on frame numbers is credited to John LaCroix, a former NAPCA member who published his research in AutoMag, NAPCA's monthly news letter.
Regards,
Burgess Mason