I would guess that, in the beginning, the Mark I circuit was a work-in-progress. It was a kind of evolving prototype so it is a gambling you are taking. Late Mark Is were more consistant.
You can, anyway, buy it and send it to Mike Bendinelli for him to tweak it to the sound you want.
I have a head in the s/n 12XX signed by RCS which Mike told me it was a "very nice puppy". He knows I like Santana very much and he offered to tweak it to the Lotus sound (live album) and he also suggested to mod the GEQ and place it after the Master, where it is much more useful.
I cannot be happier, this amp has a life on his own. Thank you, Mike!