Brendan - I'd venture that it probably is in fact the original transmission for the truck. But either way, that serial number puts it in the "close enough" category to say it is IMHO.
Have you ever posted the truck serial number/data plate info? I peeked through your previous posts and couldn't find anything on that. If you can share that info, we can do a little further decoding to better ID where it was built and other related details.
