Mail in sets. I never did any, though I remember fondly seeing the advertisements on packages and inserts through the early 90s. You usually had to send in proof of buying a number of packs or points or similar cards, and maybe have to cover shipping and handling with a small fee in order to get some mail in set.
They don't scan that well, but these are SPx Top Prospects hockey cards from that same year. These are interesting in the the SPx gold foil part is actually just on the one layer of card. These are thick cards, and can think of it as one card that is regular size, then the top part with the player picture is the same size, except cut out around the gold foil part.
Very interesting design. There is also a SN part in the bottom left where, for SN cards/parallels, the number is simply stamped on there and looks "boxed".
Interesting pick ups today all around.