Its not unusual just very very old!
I think its an old Dual Diaphragm Downey, Andale took them over (I think) used to carry parts, the gauge will have a little copper pipe on it with a washer and rubber sleeve that is compressed by the locking nut - I'm far from sure that those are still available.
If its working fine, if its stuffed its probably not worth the cost of repairing.
But ask Andale if you want more info.
Mark
There is usually a little knob on the end of the brass rod (far right of first photo) you can probably grab it with a pair of pliers to adjust.
M