Looks like Jeff has done better research than me, and found it was used on ALL 1970 Mustangs and Cougars with 351C 4V and auto transmission.
Dist-O-Vac Application? - 351C 4V | Vintage Mustang Forums (vintage-mustang.com)
The system has a whole lot of complexity just to disable distributor vacuum advance at speeds below 20 mph (and if temp is above 60 deg). The result is retarded timing for more complete fuel burn, but also more sluggish acceleration below 20 mph.
Ford was already struggling to meet emissions standards with the 351C the very first year it was used. The free-breathing Cleveland was doomed from the start.