Byron BY102 and BY101 wireless doorbell set.
The button / transmitter doesn't seem to work, even with a fresh better, so I don't know if the receivers/bells actually work.
I know it's possible to pair some Byron buttons with additional bells, so this may (or may not) be possible with these models, if anyone wants to try adding a receiver to an existing bell-push.
(Likely to go for recycling if not spoken foe in the next week or so.)