I think this engine, partially dismantled, was pictured on this site last August or thereabouts. I don't recall any other info about it and had assumed it to be OHC. Looking again I see that I got that wrong, and it has its rockers high above the head. While this may allow higher lift it would seem to lose the advantage of the upper vale guide as it seems the rocker must work on the end of the valve.
Can anyone find and link to the last photo as I can't manage.
That outfit belongs to Dave Mallows and was very successful racing until the last year or so. A dam fast motor plus DM can ride a bit too !
Dave has a clever mechanic called Mervin who did all the work including his own heads. Hardly any Vincent left but she revs to 8k and was a race winner against well developed Weslake and Imp power machines in CRMC.
BTW they found that the bike cut out under braking and had to use a 'U' shaped rear inlet manifold to point the carb backwards to stop the float depressing - problem solved.
The Mallows/Hackett outfits are rightly famous in road racing circles. Some of the innovations included g 50(matchless) valves+pistons, valve actuation from above, lots more also. Best wishes to all involved, Roy.