Syncing between machines with different versions of Unison

I have a small Debian ARM box with unison-2.40 installed. And a Ubuntu laptop which seems to have unison-2.48 I want to sync them with unison. But the the Ubuntu (which is newer) doesn't seem to recognise being explicitly asked to work with 2.40 (using -addversionno ) Is it possible to persuade 2.48 to pretend to be 2.40 or is there a Ubuntu package with 2.40 in it so I can sync these two machines?