I'm confused - are we talking about a 65 or 66? The original post mentions May of 65, so I think we should be focusing on 1965 right?
The 1965 MPC calls for part number C5GY-12127-A, stamped as C5GF-12127-A on the housing for Mustang & Fairlane with 4/B carb (except special (aka K code))
Remember that the MPC quite often lists a reduced number of parts to reduce inventory requirements, so unrestored car information is your best bet.
Since we have not heard from someone yet - in regards to an unrestored 1965 A code example, and since the 1965-72 MPC was mentioned, I thought I might look at the single year MPC for your car.
Across the whole 1965 Ford lineup, there are different distributors for auto vs. manual trans, police/economy/special, transistorized ignition, but nothing in reference to DSO or emissions.
I don't see any evidence that DSO would play a role.
The 1966 and 1967 thermactor emissions certainly did play a role in distributors and more. 1965 was a simpler time I suppose.