Yes, hardened filing jigs were used quite a bit. I don't know exactly when this became common, but history tells us it was Whitney that had the idea for interchangeability. I'm sure others might have used it prior but never thought to make an issue over it. Jerry brings up an interesting little factoid. While our arsenals quickly adapted Whitney's method because interchangeability in martial arms just makes all kinds of sense, the English held out with hand fitted martial arms at least through the 1850s.
Unless the lock is a detailed type as most fine English locks are,
I would think that careful freehand filing would work.I have done
it but that was YEARS ago.
The one job I did that I was proud of was a left and right pair of
Stanton style locks with fishtail bridles. I made long guides and
filed them face to face at the same time.I don't know if the locks were
ever used in a build or not.
Bob Roller