The Amherst A1 rear sight sets are ok to use but you do have to be cautious when purchasing them. Out of 11 sets purchased three sets had poorly machined windage drums that looked terrible and I ended up replacing them with nos Colt ones. And one other very important fact about the Amherst sets is that the ones they have been selling for a while now are brand new recent production sets and not original nos USGI sets from 1974 as they are representing them. They are using newly machined parts and packaging them to look original as copies of the very first USGI sets they were selling in the beginning. I guess they figure that the USGI type packaging and copy paper inserts help sell them easier for a higher price. I believe the windage drums are actually the same manufacture as the most recent Sarco sets which were half of the price when available. On the bright side they are nice people and they are selling a much needed product in the market, just wish they were not misrepresenting them as USGI from 1974.
You do also have to be careful with a lot of the used sets being sold on GunBroker, eBay, and elsewhere as a lot of the parts are wore out or damaged and not worth the price you pay for them.