Hi and
To the forum, though I'm afraid the parts requested a tad thin on the ground ,as in "rocking horse shit " I'd try the likes of draganfly ,burton bike bits or any other BSA parts dealer . Perhaps auto jumbles as well. The 2 splines are different lengths and have a peg on the end that fits into a hole on the gearbox plates .
On the this great forum you never know so I hope someone can help you out .
By the way we also like to know abit about you and your bike, how you got into old BSA's etc and any piccys ,best of luck Bob