[Eric72]

Here's my uneducated, lay person opinion:

If this were a factory error, there would likely be other examples from the same sheet. It seems unlikely the other 131 examples (based on a 12x11, 132 card sheet) disappeared, leaving only this Mantle.

Additionally, it seems as though the "border" of Mantle 's card was printed on the Ted back. Since Mantle's border was white, this couldn't happen at the factory. The white borders of '61 Topps cards weren't printed. The white borders are created by the absence of ink in that area. The white is the portion of the card stock upon which nothing was printed.

This looks (to me) like a fantasy piece.