Someone feel free to chime in of course, but this has me thinking I do remember reading somewhere that if you mailed or called Topps you could just buy sets of cards from them, directly, in New York?

I remember reading somewhere else, that you could've done the same with the Red Heart Dog Food set.

I think one thing about the "Good Old Days" of the Hobby, that we sorely miss today was the simplicity of it. There weren't 500 different products or sets. There wasn't any sort of fancy, shmancy parallel, refactor, printing plate etc. There was just what the main company or companies put out, plus a few regional sets.

Red heart
 

You could mail in for them as late as the early 70s. A friend had pages and pages of RH Mantles he wouldn't sell or trade.