I was looking for something else at Google Books ... and found this snippet from 1959. Printers' Ink, Volume 266, page 44.
"Berry Pink Industries, the leading maker of marbles, has switched to postcards as a substitute for samples. Marble can weigh as much as a pound per bag. Berry Pink's line became too large for salesmen to carry around and too heavy to send ... "
https://books.google.com/books?id=fN4TAQAAMAAJ&focus=searchwithinvolume&q=berry+pink
Interesting to have a date. I would have guessed a few years earlier if they're the ones I'm thinking of.
Anyone have any of the postcards?