here, but decided to also add the update in as a new post.
The thumb safety on a 1911, when pushed up to the 'on' position,
rotates a piece into place against the legs on the bottom of the sear,
locking the sear nose into the full-cock notches(also called hammer
hooks) on the hammer. A very positive safety.
Considering the number of people making parts for 1911-style pistols,
it's common to have to do a bit of fitting on one of these. Rather than
duplicate a couple of very good pieces I'll refer you to the M1911
Forum site: a very nice piece using a cutaway to show exactly how the thumb and grip safeties work, and fitting the thumb safety here.
I used a stone instead of a file(takes a lot longer to mess something up
that way, so more chance to spot it if you're messing up) so it took me
longer than it should; in any case, works perfectly.