But do you ever see
If so, the manufacturer's serial number should suffice in identifying the equipment as yours (assuming, of course, that your records include the serial number)!
Another method I have seen (and have sometimes used myself) is to make some internal markings (or labelling) inside
each unit when next opened up (for battery changes, repairs, or whatever).
But all such efforts are futile if you never actually lay hands on the equipment again (which is the usual case, in my sad experience).
Meanwhile, how did you get on with GS-1