The Colt was a repackaged Meuschke. The older Swenson style retention using a flat paddle extending under the grip is still the most common. The Ed brown, the C&S, and the older Wilson all use this style retention. There are some newer variations than the two mentioned. The (now discontinued) King, the Kimber, the new Wilson and the Blackhawk all use the hammer pin to retain the off side lever. The new Wilson and the Blackhawk use a unique split shaft with each half extending all the way across from one side to the other. Any of them will work. I prefer the new Wilson or Blackhawk. They are well made and offer the most durable, stable shaft design. I don't recommend the Ed Brown. I've seen far too many break and heard of many more from pros that I trust.
Fitting -
Lifted from a post I made on another board (with edits). This is my method.
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The safety must be fit (as needed) to the frame, the slide, and the safety detent plunger as well as the sear.
First, examine your safety to insure that the pin is straight and square to the body of the safety. I've seen some pretty bad ones come out of the package (Ed Brown). Either straighten it or return it for one that is square to the pin/body. You may have to stone the back of the shield (flat portion that rides the outside of the frame) to eliminate any casting irregularities that will prevent it lying flat and/or scratching your frame. Do not remove the radius at the root of the pin.
I also check the diameter of the pin holes in the frame and grip safety vs the pin diameter on the safety. Pin diameters I've seen vary as much as .009" (old style Wilson is noted for being large) and can effect the function of the grip safety at the extreme ends of the spectrum as well as cause friction drag on the thumb safety if there is not enough clearance. If the thumb safety drags/binds when inserted through the frame and grip safety with nothing else installed, you've got to address that up front.
Then, strip the frame of all parts so that you can insert the thumb safety by itself and observe any interference. Insure that you remove the safety detent plunger assembly. You want no possible resistance from anything except the frame itself. Fit to frame to insure that it is seated fully and smoothly moves fully from on safe to off safe. You may find the notch on the underside of the safety lug is too narrow and binding on the frame window. Relieve the face of the lug to widen the notch ever so slightly with a safe side needle file if needed. Remove only enough to allow the safety to clear the "window ledge" and drop all the way down. .0005" -.001" of clearance is plenty. More is too much.
Then install the hammer/sear/disconnector and fit the safety to the sear to insure no (absolute zero) movement of the sear/hammer while on safe. Insure that the safety is fully blocking the sear while in the up position with the slide on. Do not fit the safety to the sear with the slide off as the safety can sometimes over-travel upwards without the slide resulting in a poor fit to the sear when fully assembled. Check fit of the safety upper blocking surface in the slide notch while on safe. Adjust fit as needed.
Then remove the hammer/sear/disconnector again, reinstall the detent plunger assembly, and fit the forward face of the safety to the detent plunger for proper "feel" when moving on and off safe.
Finally, blend/contour the back edge of the shield to the frame to avoid any sharp edges/discomfort and to enhance appearance.
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A thread by a pro -
http://forums.1911forum.com/showthread.php?t=290280&highlight=thumb+safety+dent
