If it's been there 30 years then it could be good and proper stuck so that you have to destroy it to get it off.
A piece of 2" rubber hose that I just happen to have beside me (on it's way to the dump!) has a wall thickness of 5mm so you could measure the outside diameter of yours and deduct 10mm.
I would not be surprised to find that they used standard sizes - i.e. 3/4", 1", 1 1/4" up to 2" (or more). Cheap
calipers and careful measurement should do the trick.