This was an insert inside packages of Kraft Cheese Singles in Canada. It measures 3.5'' x 6'' and opens along the edge to measure 3.5'' x 12''. The paper stock is extremely thin. I'm not sure what I would compare it to, a magazine page is the closest thing I can come up with, but it's definitely a different material. You could choose from 10 different players, and needed to send in 4 UPC's and $8, or 6 UPC's and $4.
I've never seen the actual Wallach T-shirt. In fact the only shirt I've ever seen show up is a Steve Rogers shirt, and he's not listed as an option. So I'm guessing this offer was an annual thing for awhile, with a rotating selection of players. I'd be curious to see other fliers, and if the shirts changed pictures from year to year, or if it was completely different players each year. The absence of George Bell and Andre Dawson makes me suspect they didn't use players more than once.
Below are the pictures from the ebay listing for the Steve Rogers shirt. I can't say for certain it's one of these Kraft shirts, but it certainly appears to be. When I put stuff on ebay, I try to use a clean, solid background for my photos, such as a piece of poster board, or clean table top. This seller didn't go that route, and it's tacky enough that I hesitate to even post the photos. I cropped out an absurd amount of bedspread. If your curious, the seller wants $39.99 for the shirt.