I've only seen one "sub panel", it was in a "submarine", so it *was* a "sub panel".
The question, I think, is when did the NEC require the neutral to be bonded 'only at' the "service equipment".
Without going back and checking through my old codes, I'm guessing that it was 'aways' that way, just not clearly stated as such. I' think I recall that the wording was clarified in the 1920s, with continued clarification in subsequent codes as needed, ending up with the current wording.
Regardless, though, of when the code clarified the wording, it is not safe and should be written up when found - however old the house is.
