Gearbox identification

The '68 Norton project (identified through engine and frame serial number) I've purchased has a gearbox with the following stamped on it 307723 Z. Out of all the Norton parts books I've looked through I can't find what year it is from or if it is even a Norton gearbox.
Looking for help identifying year and what bike it is for before I start purchasing parts I need for it.
Thanks

Don’t know what the Z is but 307723 is an early
1974 Mk2 850.
I have one within a few numbers of that and it has Sep 1973 build date.