He had the 1975 (w/ gold knobs auctioned) LP. That was purchased in 1982.
He got the 1973 (w/ black knobs) later; it seems post Joshua Tree.
It appears that the '73 is the on in The Fly video.
Perhaps Dallas decided to label them (similarly the Black Strats) around that time.
Quite possible the numbers were somewhat arbitrary - in that he merely put a 1 on the first one he grabbed and a 2 on the second.