there are so many things that could be happening here that I don't really want to tell you to just start filing somewhere without actually seeing the part .... your sight/base should be orienting itself on the sloping surfaces of the sight & base dovetails (male and female respectively going forward.)
~however~
You have a 60^ file with a safe edge? Use some layout blue (a blue sharpie will do as well) and make sure the sight base is flat and true, not marred - just clean it up, don't remove lots of metal (!!) then relieve the top (female) edges just slightly like a couple thousandths, just enough so they don't contact the inner shoulders of the sight base you can do the same to the bottom edges of the (male) sight (see image). Don't file either parts mating/sloping surfaces
Now, I can't stress that enough, SLIGHTLY - one swipe with your diamond hone can probably remove a thousandth or more material, one or two swipes MAX on those corners, again not trying to fit it here just removing a variable.
then continue to fit that part as the video described.
if you are still not straight & you have not removed any material from the mating surfaces ~ your sight base is either tapered or someone has filed it out of true.
in that case you could:
File the sight so that you can get it in the dovetail and line it up straight by tapping it. - it will still be pretty loose.
- drill 2-4 holes in the top of the sight so you can peen (peen not pin) it in place (peening the edges of the female dovetail down)
- drill & tap 1 hole in the sight so that you can line it up and insert a grub screw to hold it in place - not a terrible idea to punch locate a divot in the sight base as well
both methods will work with the 2 sight bases you already have, since they don't fit as f now. you only get a few shots at peening the edges of the sight base/female dovetail, so use with caution.
but that's all a lot of futzing around ~ a decent gunsmith should be able to fix that up in about 20 minutes.
