Many thanks for your reply.
That's confirm we have to find rear hubs in 126 or 130 (max) size ("over luck nut" dimension is the right term) for this bike.
You know certainly this document where we can find complete information for the Exage range :
https://www.retrobike.co.uk/gallery...mano/NewFamily1989/Pages+33+_amp_+34.jpg.html
In practice, it looks like most of 7 speeds hubs sold are in 135 oln size. At least here in France for 21 speeds MTB sold mainly in the early 90's.
In classified ads, 18 speeds MTB are often (almost) very low quality bikes, with TYxx or noname components ...
Have you noticed that is exists 2 variants of Exage Mountain ?
I found (on ebay) dark grey parts and others in bright alu (as explained also in the shimano catalog above)
Moreover it exists also 2 different rear u-brakes : A ref BR-M451 (plastic covered) and a ref BR-M452 (full light alloy) !
That's why it is not easy to find a homogeneous set of "Exage Mountain" parts coming from different ads ...
PS : my seat post is stamped 26.4
It fits well, but it is not the original one ; "Italmanubri" brand ...