In 1975, Christmas stamp production just could not wait for the Postal Rate Commission to approve a 3 cent rate increase. As a result, for the first time ever, the USPS produced non-denominated stamps. The value of the stamps would be 10 cents or 13 cents depending on when used and what the commission decided. They were sold for 10 cents each.
As you may know by now, my dad was a letter carrier. He was also a stamp collector. From the 1970s (or earlier) through early 1990s, he bought 3 sheets (plate blocks, actually — I’ll address those another day) of every stamp that was released (give or take a couple): one for him, one for me, and one for my brother. When my dad died in 2007, I received my set, as well as, a lot of extra stamps. As luck would have it, I have several plate blocks and a few 4-stamp pages of this very stamp.