M5/28 c ...The C being a sub category for this entity / producer / supplier / retailer/ distorubutor by the name of Julius Dinnebier.
Not all M5 marked snap clips had a sub catorgory..but some did.
M5a - Buttons
M5b - Side Hooks
M5c - Spring Clasps
M5d - Fittings and mounts
M5e - Single tongue buckles
What I do find missing from that M5/28 producer / etc ... is that only a, b and d are listed as having that sub letter on their wares.
IMO Spring clasps is correct for your snap clip ...so when these numbers started first appearing ...they started out with ...EX : .... M5/44 or what ever other number ...and later on it appears the directives may have caught up with the rest of the RZM entities and some other RZM numbers are appearing with these sub category letters.
The time frame for this snap clip is around December 1935
The dagger is a nice find
Regards Larry
