Interesting mixture of dates. X517 rims came out in 98, canti bridges died out in 96, 39cm rigid forks hung around a lot longer than maybe they should.
I had a TotalBike in 1998 with a test of a Ribble, I think it was an R4, although the tubes were round on that and yours don't look round. The usual thing - Ribble or Merlin vs big brand names. The Ribble was by far the lightest, by far the best equipped, had the best frame, had hand-built 517 wheels, but they hated it. Ribble had clearly had to fit a P2 fork in order to get to the price point of the group test, the frame needed a longer fork, so the head angle was 72 degrees, the handling was twitchy and of course none of the testers knew why. So they panned the bike. Put a decent fork on it and it was by far the best bike there, but the testers didn't know that. Soon afterwards, I noticed Ribble stopped selling mtb frames/builds and stopped advertising in mtb magazines!
Looking at yours, it does look as though it has quite a lot of bottom bracket drop, which is an indicator of fork too short for frame, so possibly a set up similar to the TotalBike R4. Obviously you like it as it is, but I just wonder whether you ever thought of trying it with suspension? A Z2 Bomber might suit it nicely.