I forgot to reply to this. This, apparently, wasn't an odd practice. I know of one instructor who was promoted 2nd-5th dan by letter, by an instructor he hadn't physically seen in years. 3rd-5th were (and still are in the system) considered more honorary ranks. Second was (and still is) seen as the last technical (skill-based) rank. From what I understand, when folks were distant from any possible instructor who could rank them up, if they were active and producing students for long enough, someone back in Japan would rank them up in recognition of the time and effort they continued to put into the art. I don't know how common it was (nor if it still happens, now that video is so easy to exchange), but there are examples enough that I doubt it was surprising.