Thanks jaf72, and welcome to the forum.
I haven't discovered much new since first posting this. In fact I was just going through them today so I can present them at 'show and tell' night at my stamp club tomorrow.
The envelope with the postage due stamps contained a rather thick letter where the writer outlined that the censorship rules had now been lifted and he was able to tell more of his 'story'.
Turns out he was a supply officer and his company had landed in Okinawa on the USS Admiral W. L. Capps.
Here's the Wiki page about the transport ship.
http://en.wikipedia.org/wiki/USS_Ad...%28AP-121%29His last letter in 1946 talks more of his wartime experience, and refers to being sent into China and Korea after the end of hostilities, to what end he doesn't say.
Mostly though, he writes about the boredom. In one letter most of his motor pool has been taken away to make room for a rec hall for the enlisted men.
In all, an interesting look at the time, and a nice collection of US military cancels.