73' is a bit before my BP experiance,but T/C has the same lock
(coilspring).
The thing that holds the hammer on and pokes thru the lock plate is the tumbler,and sadly many folks don't lube that thing and it wears.
I'm not so smart I know this off the top of my head,I have a picture of a lock I look at every time i need to identify a part.
Anyways, check with Dixie Gun Works and Track of the Wolf. Both have a easy to find web sites with e-mail contacts and answers for parts quearies like yours,it's free.
I hope this little bit helps,if I can help further,just post a return,good luck.