Oh, that's cool!
The missing link between the breaker-point distributors and durasparks, combining a breaker-points housing and a duraspark reluctor and magnetic pickup.
Have seen this distributor only three times, and only one time in person, and believe it to be quite a rare item. All beared D3 casting numbers, stamped from late-'73 to mid-'74. The pickup base as well differs from the common duraspark unit, as it fits inside the smaller breaker points housing and has a frappant similarity to the V8-durasparks.
(While I am not absolutely sure of the nomenclature here, this might be the "true" DS1, while the small cap of what we have thought of as a DS1 might just been an early variation of the DS2. Purely hypothetical, of course.)