I now have possession of an M25 Mono ... so not an MS ... I thought it looked slightly different! I'd guess it's the forerunner of the MS, so probably quite old.
The chap said it works OK and he'd used it for irrigating his garden with grey water. Nice chap, bit of a home engineer, lathe, band saws, table saw welder etc, etc and a confirmed hoarder of things mechanical.
I noticed on getting it home the thread in the suction post is very badly rusted (had the same problem with one of the MMs I have). I think I may have a suitable tap ... do you want me to clean it up?
Let me know what you want me to do with it. I should be attending the BBB, but as it seems to be a long one, we maybe need to liaise to make sure we don't miss each other.