In the mid 1980s builders were cutting the carry handles off of A1 upper receivers and screwing on scope rails. I didn't bother with that mod at the time since my eyes were still good enough for iron sights.
It may be sarcasm but, I saw something similar happen. I was in AK in 1976 with the 23rd Cbt Hvy Engineer Co and saw a guy hop down off his D8K Dozier to take a ****. He leaned his M16 up against the front of the track and forgot about it. You probably know the rest of the story. A D8 does a little more modification than just making a flat top out of it.My guess would be that the first one was created on February 6th or 7th 1968 by the tracks of a PT-76. That's why you never saw anybody make flattops for decades, it brought back memories of Vietnam and Lang Vei.