Here is an example from 1972:
I'm not really cataloging what was packaged this way but am trying to cipher what was shown on the underside of these. Here is an example from 1974:
Another intriguing tray back set of the era, but is it from Topps:
I'm not sure what kind of tray these are from though; they don't seem cut down and the rounded corners look consistent.Those are being offered by Columbia City Collectibles by the way as Topps product, likely based upon the first five digits of the bar code.
1980 Baseball was the last product issued with a cardboard tray. Topps switched to a floppy cello in mid 1980 and then by the time 1981 Baseball came out the wax packs had been replaced by clear cello's, usually with 12 cards (and gum).
There must be a lot more tray back cards out there; I plan to keep looking and if anyone out there has some scans, send 'em in!