Up to now there are no exact production figures of 1200 OSI or Neckar St Trop cars available. The figures vary from 105 cars in total up to 280 Spider and about 70 Coupés. Even the VIN gives no clue because OSI appearently ordered chassis from the Fiat factory always just in time to their own production, so there is no continous series of VINs. But OSI had, as many other low-volume manufacturers, given it's own production number to their cars. This number can be found on various parts of the car, e.g. the inside of the door panels or at chrome parts. But first of all it can be found on the left front wing, next to the battery.
If you assume that all production numbers were actually used, it is quite likely that OSI built a little more than 700 cars in two years. The lowest number in my registry is the OSI Spider 00024 and the highest is 00733 – also an italian OSI 1200S Spider.
So when "Ruoteclassiche" publishes a total of 280 OSI Spider and some 70 OSI Coupés, it is quite likely that their figures only refer to the Fiat models. It is safe to assume that at least the the same amount of Neckar models were produced in this period.