Colin, I was involved in postal Dip many years ago, in the early '80s. Sassenach and SLOTerp are correct with regard to the length of time. Think of two years for a game. Because seasons were every 3 or 4 weeks, you could actually accommodate several games at one time without getting overwhelmed. Back then, it was common for games to get an official "Boardman" number that a volunteer would manage. Game results were cataloged and helped produce a lot of the stats that people have since referenced when talking about "most likely to win" or "to lose" countries. I don't believe they are used much, if at all, these days. Here is a link to a nice history of the "Boardman Numbers" by Rod Walker: http://diplom.org/Zine/S1997M/Fitzpatrick/Bnumbers.html
Of course, you spent a lot of ca$h on paper, envelopes, and stamps for postal play. And you often had to pay a subscription to the 'Zine that the GMs published that contained game results, editorials, and other stuff. Then again, you pay for your Internet access, too (well, most of us, anyway). But there was time to get involved in devilish plots and deceptions. One favorite was mailing phony letters from the city of one of the other players. You had to know somebody else in that city and send them the phony letter/envelope so they could mail it for you. Some players took to coding their letters to prevent that kind of deception. But it wasn't as bad as Chess-By-Mail, for example. Even Gunboat was done by mail. I suppose that was the simplest and cheapest, as it required only 1 letter per month. But Gunboat is a waste of time, I think.
In addition to Jim Burgess, there is Douglas Kent, who edits Diplomacy World. Email him at diplomacyworld at yahoo DOT com. He was around back then. He also runs the Eternal Sunshine zine, I believe.