Many factories of older times have, or had, only one file copy. Consider that only in recent decades back to 1995 or so were CAD, scanners, and computer systems cheap enough for routine use by everyone involved in a small company, and that for the the older products either done on an obsolete computer or done by hand, it would not have been a priority to copy it into the computer and the engineers couldn't be bothered to do it, 'document control' did not exist, and the regular minions having no access to resources to do it with. Lost manuals are endless.
IFI model 400 amplifier (six 4CX250B) - lost in fire.
An Ultrasonic cleaner here - lost in flood.
I am sure your right about a lot of the smaller companies.
I always felt compelled that once I took someones money to maintain records for at least as long as I am alive. I ran my own electronic design and manufacturing business for over 10 years and I have every bit of documention should a former customer need it. That also was before we had very easy ways to electronically store things.
The Hamtronics stuff is simple enough that I can just come up with a schematic for the transmitter from the assembly I showed. Not a big deal that I get this from anyone but it never hurts to ask!
Joe, GMS