Proper term is edge stain--referring to the process of sanding the edges of the text block and dyeing them. In this case red. It was done to protect from dust, and dropped in the 70's due to expense.
Three digit product numbers, the lower the better with "cartoon" style cover paintings can be worth some money. There are collectors of Harlequin men's adventure books (later Gold Medal) as well as the romance.
One title, in particular, is worth significant money. It is a Nora Roberts title, and she calls it the worst book she ever wrote. A bit of research should turn up the title.
FWIW: these are popular titles. When our FOL was operating, we'd get in thousands of them. We'd try and sell only the most recent titles singly, the rest we'd bag up and sell at 20/$1.00.
I'm hoping we get back to sales soon.
