The Ballester-Molina is a funny pistol. It looks so much like a 1911 but it is actually more closely akin to the Star Model P. As such, unlike it's Sistema cousin, it is not under a Colt license and was manufactured by HAFDASA (Hispano Argentino Fabrica de Automoviles Sociedad Anonima, a civilian car parts company in Buenos Aires) whereas the Sistema 1911 was manufactured by DGFM-FMAP (a government-run manufacturing facility).
I bought a cheap beater Ballester-Rigaud (the earlier version of the Ballester-Molina) that had no sights on the slide. I was able to put a set of MMC 1911 sights on it. The front sight went on fine, but the dovetail for the rear sight was too large. The sight went in OK but was too loose and would fall out if the pistol was turned sideways. I was able to wrap a piece of .005" shim stock around the 3 sides of the dovetail and tap it into place with a hammer. This allowed the sight to technically work but you can visually see that "it ain't right". I relegated this weapon to the status of "truck gun" because it looks so bad but works so well.
You might find that a set of 1911 sights goes right in your gun but I'll bet you run into the same thing I did. Take the slide with you to the next gun show and see if someone has a used USGI rear sight that they will let you try to insert into your slide. If you can push it in with light finger pressure then the sight will never stay put under recoil.
-Lee
