dfx
Contributor
Maybe the story you're thinking of was mine. Before I had the good sense to convert my originally-DIN regs to yoke, I had the experience of renting a tank at an Atlanta dive shop to use at a lake, and finding the threaded opening to have been deformed to the point where I was unable to screw my DIN reg into it without exerting what I felt would be too much force. The threads themselves looked okay, so my guess was that the fitting had gotten banged against something, deforming it ever so slightly. In fact, I believe my wife and I had rented four tanks between the two of us, and another one of the four was apparently also slightly deformed, such that the reg did not screw in smoothly. I was fearful that forcing my reg into a non-circular opening might damage the threads of my reg. I can see why shops might believe DIN is not a good option for their rental tanks. For the record, all of this shop's tanks nowadays have converter plugs, though if you rent a tank for use with DIN, the shop employee removes the plug before you leave the shop and keeps it. I have not tried to remove any, and I don't care whether they are or are not rusted in place, since my regs are now blissfully yoke.
Before I had my own regs, I used to use rental yoke regs, and that one time I wanted to go diving and one of the yoke tanks had something wrong with the valve. I think it was the o-ring, so I tried to change it with a spare one that I had, but it was a different size and so didn't fit. I had to skip that dive.
Obviously my anecdote trumps yours, so there.