Wow, I had one of Martin's very early ones, exactly like yours - a 5 headless, amboyna facing (think yours is that?) - had mine built back when he was working out of his flat in NW London (may have been Willesden, was a long time ago!). IIRC, I discovered the early Sei basses through a player I knew who was a part of the early '90s north London acid jazz scene. They were a seriously hot ticket for a while.
I had Bartolinis fitted to mine but otherwise really looked ther same, sadly don't have any pics to hand.
Was a truly epic bass, beautifully built and very nice to play - as you say a very clean musical and balanced B, I mean transition across B to E sounded seemless.
Sold it on a whim... like we all do.
As a side note Martin also took my '64 Precision in for a refret at that time (around '91 I think). I know he took very detailed measurements of the neck which I think he referred to later when he made some P style basses.
Enjoy it... and try and keep it!!