Hi There
This looks like the 90+ that is on display at Sammy Millers Museum. If so I am pretty sure this is the one has been modified in the past to have chain driven camshafts, for some reason that is beyond me, maybe it had shed some teeth of a cam wheel and rather than replace them, they opted for a different solution, I don't know. The 4 bolts in the cover maybe for mounting some sort of a chain tensioning device.
May be contact the Museum to find out a bit more of the history.
Cheers Steve.L