I am not sure what you mean by a 'Works Cooper" or "Works Rover". The only Rover Cooper to have an oil cooler as standard was the 1990 RSP and that was a limited edition. John Cooper Garages had a series of S conversions for carb, SPi and MPi models. The carb and SPi versions of the kits included an oil cooler but the MPi version did not. The "S Works" was introduced in 1999 as a Cooper conversion claiming 90bhp for an MPi. In the same year there was also "S Works" upgrades to earlier S conversions for both MPi and SPi models with the SPi version claimed at 86bhp. The S Works upgrades did not include oil coolers but the SPi would already have one from the Si, Si plus and Cooper S kits for which the S Works kit was just an upgrade. I am guessing from the reference to the plate and certificate that your SPi has had a John Cooper conversion of some sort.
My understanding is that it rolled off the production line as a rover cooper - but then the first owner took it to Cooper works for a cooper conversion - that make sense?
Yes. That makes sense. However in the early days, from 1991 on, the Cooper 'S' conversions could also be carried out by Rover dealers. The original Rover warranty would be retained both for dealer converted and John Cooper Garages converted cars. I am not sure when the Cooper kits stopped being available as dealer fitments.
In 1995 John Cooper Garages started producing their own car. They called it simply the Mini Cooper S. They would order a new standard SPi from Rover, register it brand new to the customer in the customer's name and convert it to their own Cooper S specification, which was a further development of the earlier Si and Si plus kits. There was also the limited edition (of 35) Grand Prix model with further enhancements.
The Mini Cooper S model had a distinctive rear quarter Cooper S decal, which is different from the Cooper S decal on post-conversions. Plates on the John Cooper Garages produced cars also had a car number rather than a conversion number, but I am not sure when that practice started.
My confusion with your original question was the term "Works'. I didn't intend to be pedantic. It's just that the word is more associated with the S Works of 1999 and that John Cooper Garages was the more normal term before that.
