I have two machines. One with unison v. 2.40 (Debian Jessie) and a new one (Debian Stretch) where I installed package
unison-all, which brought in unison versions 2.32, 2.40 and 2.48. However, I was unable to make my 2.40 work against the new machine, since it only presented v. 2.48. No matter what I do, I get:
expected "Unison 2.40\n" but received "Unison 2.48\n\000\000\000\000\017",
I worked around the issue by removing
unison-all and installing v. 2.40 explicitly, but I wonder what to do if I ever want to use two concurrent versions of unison on this machine.