Pretty big for a stamp, but pretty small for a record.

I polled a couple of AIs real quicklike. They're telling me that the label dimensions for vinyl records range from 75-100mm.

That means that these stamps were no larger than the label on a typical record you'd put on a turntable. That would make them nigh-unplayable in modern systems.

In modern turntables, each tonearm has a limiter that prevents them from straying past the runout grooves. The runout grooves are designed to signal the end of media. Typically you'll have some wide spacing and then the spiral ends in a completely circular track.

Older turntables would just sit in the runout grooves and play silence. You could risk damage to the needle or mechanism if the needle accidentally hit paper or the arm hit the spindle. Better-designed turntables would sense this, usually based on distance, lift/return the tonearm mechanically, and stop the playback.

So the only turntables which can "play" these "stamps" would be ones which ignore that minimum size requirement. Perhaps some really high-end systems have a disabling switch, but consumer-grade ones sure didn't, in my recollection.

IIRC one of the other articles mentioned that many turntables could not actually play these record, or perhaps only the larger ones.

Now I wonder if these were ever really used as actual postage stamps? Besides the size and weight, stamps need to be "cancelled" by, well, stamping them with another kind of stamp (https://en.wikipedia.org/wiki/Cancellation_(mail)) before an envelope goes in the mail, and I can't imagine how that would work with these...

EDIT: there are also first day covers using these stamps available (https://boingboing.net/2022/07/07/look-at-these-cool-1970s-p...), and the photo shows that it actually works, but not really well, especially with the black one...