Just seen this thread, great read and fantastic that you're keeping the 89 Explosif close to original spec, the reason I'm posting is that I was looking out for my old mk1 that I sold some time ago possibly in 1990, I know it sounds silly but I think you may have it, I removed excess metal from the top of the head tube to shorten it, if you look at a standard Explosif you will see they have longer head tubes than yours, I can't imagine that many if anyone else preformed this mod, my frame was a 19"