more or less hydro lock.
I may be full of it
GOOD ONE !Hydra lock would be leg cuffs for a Japanese horror film monster.
Don't be offended, I'm speaking generally (and speaking from 45 plus years as a mechanic ((fifty if you count the years I was working on my 20inch stingray bicycle . . . I put chopper forks on it I made out of EMT ) . . .if you get water between the washers it can create a suction as they move and seize. they get stuck and lock up.
Or a skinny butt kid with all the weight tipped onto the rear wheel. I was eight or nine at the time. It may have been some good stuff, my Dad was an industrial steam fitter, I just used what showed up in the scrap barrel.Unless it was some exceptionally heavy duty stuff!
I just went down to the shop and measured the tubing from that period that I later used to make a tool handle : wall thickness 1.3mm , OD 23mmUnless it was some exceptionally heavy duty stuff!