Charging system suddenly quit.

To simplify a bit what Bill said, unhook the green field wire at the alternator. Use a clip lead and jumper the brush you just disconnected to the starter relay battery stud See if it charges.

If not, measure voltage at the large alternator output stud If it's very very high, perhaps more than 16-18V, you have a break in the alternator charging lead going through the bulkhead to the ammeter

If it's very low, 11V or so, the alternator is not working. Is it possible? Your replacement is either bad, or is a "dual field?" (That term dual field is incorrect, by the way, the 70 / later ones were properly termed "isolated field. They have two brush holders both insulated. Yours has one brush grounded)

I cannot see for sure what you have in that photo. The earlier (70--73 or so) isolated field units were also the "round back" case style