A pair of Exhibit test pieces:
On the left is a Mays proof.
A Salutations Holmes proof. The white borders are far beyond the normal card edges.
I love this newspaper issued 1960 Koufax:
Hallicrafters Lemon and Rosen premium:
After years of research I was finally able to pin down these nearly square smaller paper premiums:
1955 All American Sports Club.
Here is an interesting cabinet-sized version of the 1969 Shindana Ali card, next to the as issued card:
I say prototype or test because some of the cards have art that is different than the art on the as-issued cards.