So here's the scoop...
Topps and O-Pee-Chee included cards on the bottom of the wax pack boxes from 1986 through 1992
1986 - Cards from both companies were printed on the bottom of the box. There are four panels, with four cards on each panel, making a total of 16 cards. The fronts and backs of the cards are identical to the regular cards and the cards are the regular size. There are no Cubs among the 16 players.
1987 - There are just eight cards. Topps printed the cards in panels of two on the side of the box while OPC put four-card panels on the bottom. The cards are slightly smaller, measuring 2 1/8" x 3". The fronts are identical to the regulars while the backs are different. They highlight a career milestone achieved during the 1986 season. Ron Cey is the lone Cub in the set.
1988 - We're back to sixteen cards and both companies put them in panels of four on the bottom of the box. The cards are full sized and have a front the same as the base set while the backs again highlight a milestone. There are some OPC versions that have blank backs. All of the Topps card have a front and back. The Cubs were shut out on the checklist.
1989 - Things are settling into a groove and nothing changes in 1989. Its sixteen full-sized cards on the bottom of the box. The backs include a career milestone. We get a Cub this time, Goose Gossage.
1990 - All things are the same in 1990; 16 cards on the bottom with career milestone on the back. This time, for the only time, there are two Cubs in the set, Andre Dawson and Ryne Sandberg.
1991 - Same old, same old for the fourth year in a row. Andre Dawson was the only Cub included.
1992 - Big changes here. Topps ditched the cards on the bottom. OPC included four different cards on the bottom. The cards were the full size of the box bottom and featured team-centered events. None were Cubs events.
All good things must come to an end and the 1992 season was the last of the box bottom cards.