Slight correction fellas. Not ALL will work, on the earlier Colt models, at least up thru the A/2 versions no relief cut was made on the underside of the upper to accommodate the auto sear.
I own a converted A/2, purchased in the immediate time frame before Hughes went into effect. Had a dealer do the machine work and put it on his books. When I finally got to take possession of the lower I found it unreliable. Essentially it was firing by virtue of hammer follow thru........did the machine cut on the upper and problem solved.
Bottom line....if your upper is cut for the AS you are good to go. Without that cut the upper will partially or fully depress that sear and keep it from engaging the hammer extension.....a loose upper might let it work sometimes but usually you'll get the follow thru I mention.