[derefr]

Literally spooling message-filled tape onto a tape reel! I never even thought about that.

On the topic of message queues, I wonder if anyone ever designed a mechanical, asynchronous hardware IPC mechanism based on tape. Imagine: an endless-loop tape that stretches between a "sender" tape-drive on one machine, and a "receiver" tape-drive on another machine; with each tape drive locally tensioning the tape as it passes through, but letting it hang slack outside the drive; with two tape-width buffer boxes in between the two drives, for taking up the slack; and where each tape-drive has a strain gauge on its input-end capstan.

With this design, you'd never need to seek the tape (backward, at least) — the receive head would just read the tape forward at whatever speed it could until it ran out of slack in the tape (i.e. blocked on read); and the send head would write the tape forward at whatever speed it could until it ran out of slack (i.e. blocked on write.) The tape itself would be a literal, physical ring buffer.

[wolfgang42]

Western Union built a store-and-forward message switching system in pretty much exactly the way you describe (but with paper tape rather than magnetic): https://en.wikipedia.org/wiki/Plan_55-A

[robotresearcher]

Analog audio delay/echo machines work like this. They could have multiple read heads and you could move them relative to each other and adjust the speed of the tape to get various cool effects.