OMG ...
"Most of the few RR tapes I have came from radio stations-most are full of commercials sent by clients to be broadcast.The commercials would be transferred from the reels to carts."
OMG this was me in one of my first jobs in radio (KDKA in Pittsburgh)
ALL the commercials came in on reels. When I worked in "traffic" and "continuity", I had to listen to all of them, time them, make sure there were no editorial issues, and then send them down to production.
When I worked in production, I'd transfer each of them to their own cart (short for "cartridge" -- which were nearly identical to 8-track tapes, but only held enough tape inside for 30 seconds, 60 seconds, 2:30, etc.). They were a marvelous invention, really: pop it in, hit play, and it would automatically cue up to the beginning after each play.
With the exception of hour-long pretaped shows, KDKA put EVERYTHING on carts, even (when it still played music), songs.