I can't seem to figure out what this 43rd weapon is...it's the last one I need to get, but it's not in the shops, and I've done everything else.
If you have the Deluxe Edition, activate it and you'll get the Tribal Knife and a cool sniper rifle right from the beginning.
Also, there are the signature weapons, but I think you already have them.
Take a look here to check the stuff you have:
Link
Do you have the custom magnum? The one you get by redeeming Uplay award points or whatever they call it.
According to him, he has all the Signature weapons... The only thing I can remember now is the Japanese Tanto.