Scales.

All scales are earned by trading in:

All 4 Gronk's 89 Wolfe Wolfe's minions 89 Rodgers Rodgers Minions All 4 Cooper's

Once you've built all these, you'll be able to claim the 90's exchange for 91's, then exchange for 92 Dawkins and all 4 92 Masters. You'll also get to keep all the minions and three of your Cooper's.

I haven't done it yet, but I would assume you get scales for trading in the 89s/90s/91s