Industrial slant six timing.

^^^ Yes, the older alternators in particular will shows discharge at low idle but they all will to some degree. This is normal if it picks up and shows charge at higher RPM's. (If you really have to rev it to see charging, then perhaps a couple of diodes are blown, but I would not go on about that based simply on showing discharge at low idle.)

The other bolt under the distributor is under the opposite side from the regular hold-down bolt; just pop out the distributor and it will be obvious.

I would take the time on an unknown engine to check the timing marks against true top dead center on cylinders 1 & 6. Not all that hard...

Got any idea how many hours are on this engine?