The "known" M16 grenade launcher sights are:
1) Colt 'clip on" front sight, these are marked Colt-Armalite & have a Colt part number, they were delivered w/the 601 & possibly some 602's to the AF & Army, they are meant for the Mecar "Energa" rifle grenade (they're actually marked Energa on them), that's the one that the spring clip on the muzzle is for. These are the grenade launcher sights that are in the early Colt catalogs & the original AF manual for the 601.
2) XM148 launcher sight, this is a seperate sight that attaches to the side of the XM148 launcher itself.
3) M203 launcher sight, these have a seperate front sight that attaches to the M203 HG & a rear sight that attaches to the carry-handle, I've never seen a 203 on flat top upper w/a sight so don't have any idea how one would attach (before someone asks LOL!), there are M203 HG's w/the F sight attachment for the M16A2/A3/A4 & the M4/M4A1.
These are the only US military used grenade launcher sights that I've found listed in manuals, 1962, 1964, 1966, 1968, 1972, 1977, 1983, 1984, 1995, 2001, 2008 are the manual dates I have & have checked thru, they cover the 601 thru the current issue guns.
Early in the M16's service life the US military did have a number of rifle grenades "in stock" besides the Mecar "Energa", the training models of some of those other grenades came w/a training sight in the box w/the trainer grenade. Possibly the sights shown in that aucton may be those but I don't know about the longer piece, from what I've read/found the trainer grenade sights were similiar to the Colt Energa sight & clipped on, they appear to have been "disposable" items that were discarded when the trainer was fired.
There was very litlte use made of rifle grenades w/the M16 as by the time it was in general service the Army had already issued the M79 & the XM148 came into service around 1965-66 soon superceded by the M203. I don't know what the sights are that are shown in that auction but I don't think they're US military unless they're the training sights, possibly either foreign mfg or a contract for a foreign military as other than the 3 mentioned above there's no mention in any US military manuals of grenade/launcher sights.