My previous reply hasn't been approved yet, but for any future travellers: I just learned you can cutomise the name that appears by going to Settings > Sync on each device. Can't believe I didn't notice this before – it's been annoying me for ages!
Sad to see this hasn't got more votes, although I suppose it is a minor annoyance in a minor feature. I usually prefix my device names with "amc-" and have the same issue:Screenshot of Firefox on Android's share menu showing three machine names. Two ...