The old-dovetail sights set that I had bought from Dawson for my old-style T9 required the front sight to be lightly filed in order for it to start to slip in. I believe that most aftermarket sights are made slightly oversize due to variations in production.
Not to scare you, but you'd better hope a prior owner didn't mess with and enlarge the gun's original dovetail -- or no set of old-style dovetail sights will ever fit...
* Are there any file marks or indications that the dovetail was modified?
* Look close with a magnifying glass in good lighting.
If that's the issue, you can lightly peen down the tops of the barrel's dovetail where they ride over the front sight base; another trick is to use a sharp punch at a slight angle and peen the metal of the bottom of the dovetail creating little dimples/humps that increase the friction fit of the sight into the dovetail. Worse case, use those 2 prior tricks first, sight it in, then add some JB weld on each side of the front sight base's (after sighting it in) to prevent it from shifting L or R.
I had to use those tricks on an old rifle where the prior owner had filed on the gun -- and not on the replacement sight...
Old No7
"Freedom and the Second Amendment... One cannot exist without the other." © 2000 DTH