Well, as I'm in the enviable position of currently owning all three, I've just nipped into the secure vault and can reveal as follows:
1983 Gen1 - just a smidge over 17.5" but it was hanging up high, so could just be 17.5"
1988 FST - 18.5"
1988 Sport - 19.25"
Can't wait to get the Sport finished, photographed and then fook it, it's getting thrashed