Many of those units went bad due to lack of lubrication to the tach drive gears.
A very good mod is to add a grease fitting to the side of the housing near the gears, and look inside where the driven tach gear thrusts against the housing. If the driven gear has worn into the housing, it can jam and strip the teeth off both gears, you can drill and tap the housing for a quarter inch bolt and use a lock nut on the outside. This is a good fix for those distributors.
I can provide a photo of it if you need one.
These distributors can trigger an MSD 6 or higher. By the way, the pickup and replacement main shaft costs a bundle these days.
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