The front sight on ribbed guns has a tab below the ramp that the pins pass through to hold it in place. The ramp sits right on top the rib grooves, even on factory barrels, by the way. The first thing that means is you must cut the barrel back enough to remove the original slot for the ramp tab. Next that to reinstall you need to cut a new slot. That is not hard with a mill. Once the slot is cut you need to make a decision. To hold it back on with pins or use other means. The older guns used two pins that set proud with rounded ends and the newer ones the pins are polished flush.
Polishing flush means you will need to blue that whole area. Proud pins you only need to worry about the ends of the pins.
But, a bigger problem is the holes. Hitting the original holes in the tab perfectly would be difficult and hitting them a bit off with a little .050 bit would be a good way to snap off the bit. So, plugging the original holes in the tab with a pin and tapping the pin to rivet it in hole the filing it flush is a good idea. Then you need to install it on barrel and drill the 2 holes. This means the barrel must be held solidly, be both level length wise and the ramp must be perfectly 90 degrees to the drill. Then you must drill 2 holes perfectly straight though one side of the ramp the tab, then though the rib on other side with a .050 drill. IF the little drill wanders at all it will not come out the other side right. I screwed up the courage to try it one time on a model 29 barrel. Nailed the first hole, I moved the mill bed enough to be over second hole location, which kept me level and square and did the second hole using same bit, pressure etc. The bit wandered a bit and came out on the corner of the barrels rib and it is now a DUH reminder in my parts pile. About all I can do is cut it back more, polish off the markings and make it about 3 1/2". Ir is also one of the reasons, I don't like working on some one else's gun. I mess up mine I will deal with it.
I will never ever try to do that again.
It is much better to tin the tab and sight base with silver solder, get solder in new slot, then clamp ramp in position. It will never ever come off without heat. If you use a big carpenters pencil to hit the sides of ramp and barrel the excess solder that squirts out will not stick to them and cleans right off. It looks no different than one with pins polished flat.
I am going to re blue the Brazilian. The bright blue cylinder and barrel make the frame look funky.
You want to see a 5 screw pinned and recessed 45 colts here are 2. One from a 1955 the other from a early Highway Patrolmen. Both have recessed 44 mag cylinders reamed to 45 Colt
Here is a 3 1/2" made using a 1917 frame with the grip frame chopped and rounded, milled for sights, a cut down 1950 barrel and a non recessed 44 mag cylinder reamed to 45 colt then the center portion of cylinder recessed so it can also fire 45 acp in full moons. The only part of the original butt base left is just enough for the original serial number. Cut, form, weld, cut, form weld, it never ever left the gun
