They're just held in place by wedging into a dove tail (right side to the left) and usually staked (punch mark) to hold them in place, so they can get loose and fall off. Also, original leaf sites are now very expensive and have been desirable for quite a while so maybe someone 'borrowed' it.

For a shooter rifle and plinking the adjustable sights have a lot of merit. They designed it as a replacement because it functioned better with it's versatility of adjustments. I understand your desire to have original USGI parts and I agree. Just wanted to let you know the little leaf sights are very expensive collectibles (over $100) and difficult to find originals. There's probably 50 or more fakes being sold for authentic for every authentic one found, and even many people familaiar with carbines can't tell an authentic leaf sight from a $10 reproduction by looking at photos on sale sites.