They were developed in the mid 80's. Their use isn't super well documented, and the 633's Front Sight folding is awash in apocrypha. As Earrhardt mentioned, usually it's said to be for use with firing ports, but I've never heard of anyone who's actually SEEN a port that this was supposed to fit into. It's also often speculated that the sight folded to keep from snagging when concealed under coats and the like. Again, no proof has been forthcoming.
RO633 uses the folding sight and the earlier 231 handguards. It is often dubbed DOE, for Department of Energy, and is said to have been used to guard nuclear sites.
RO634 is a semi auto version of the 635 (according to the chart)
RO635 has a 10" barrel and uses standard carbine handguards. It's pretty well documented as being used by various law enforcement agencies.
RO636 uses rifle length A2 handguards over an integral suppressor, and is often dubbed DEA, as it's said to have been used by the DEA to raid meth labs. I think we had someone on here who attested to its use as such.
The prototype in my profile photo has a fixed front sight base, and a one piece aluminum freefloat handguard. It is labeled in Black Rifle II as RO663 in one caption, but there is also a model listing table with no 663 listed, and instead the gun seems to match the features for RO630, which would be the basis for the 630 line and make sense for a prototype, except that 630 is also supposed to be the GAU-5A.