Is it possible that a pre 1897 C-96 Mauser exists?
Oh yeah, they exist. Broomhandle #### time
what serial number is considered the "cut off"?
Whats the name of the store in red deer?
The serial cutoff for these are very hard to pinpoint. The RCMP lab will require a full set of digital images and will not be willing to verify antiquity off of serial alone on these.
The reason is that there were contract runs that started back at serial #1. There are also anomalies where later frames used early production parts so a Broomhandle might visibly look antique (conehammer, early production upper, etc) and be built on a later frame. Needless to say nothing went to waist in those days and a box of old parts wouldn't be looked over for new production runs.
When learning about Broomhandles you're going to have to look for other signs of antiquity besides serial number. Also keep in mind that during 1897 the design wasn't stabilized and there can be some unique finds, making one Broomhandle command a larger collector premium than another (even seperated by one digit in the serial range). Signs for antiquity to look for are; conehammer is a good start, no serial number on diagonal flats of barrel, serial number found on exterior on back of frame and hammer, no Mauser factory markings on left or right side of frame, "WAFFENFABRIK MAUSER, OBERNDORF A/N" marking on top flat of barrel.
As for the guy in Red Deer he's no longer selling antiques. AFAIK the RCMP shut him down, not sure of the reason. If you're looking to spend $10000 on an antique broomhandle (his last asking price) I'll sell you mine for cheaper than that

.
If you're looking for help on identification feel free to send a PM or post here, I'll be happy to help.
The ones that are 9mm are made after unless they used the frame so it's still antique it comes down to serial number some are some arn't
Some have been bored out to 9mm once their original 7.63 bore was shot out. Others are lucky enough to find a 9mm upper (these are hand machined and fitted so not all uppers interchange readily) and change out from 7.63 and 9mm.