ID plates were used up til the end of 68. Presumably all cars built to end of 68 production would have had an ID plate.
69 production cars did not have any ID plate at all and VIN plates were used from the latter part of 69 in anticipation of their mandatory use from Jan 70.
Don't confuse build date, which is at best an approximation for cars built up to end of 68, the date it actually left the factory and the date of first registration.
There are body numbers, car numbers, model codes and production codes. You need to have ALL of these correct for a genuine car.
Just because it has all the bits doesn't make it a genuine car.
The one way to tell if it is a genuine S is to check the production code stamped on the radiator shroud. It will be M2 06 for a genuine S body. The BODY number should also be stamped on the radiator shroud.
As well as the production code, the model codes are YKG2S2 for a Mk 1 S and YG2S4 for a Mk2 with a VIN plate (note no K). For cars built in 69 without either an ID or VIN plate ie early Mk2 S, I'm not sure if they had the K in the car code.
I think the car code was stamped on the radiator surround panel on Mk 2 cars but not 100% certain.
Mk 2 owners info needed here.
Body numbers start from 0001 and car numbers start from 501. There should be approx 500 difference between the car number and the body number, but BMC didn't get it exact all the time. My own Mk1 has a 504 number difference.
On a Mk1 S the CAR number should be stamped on the firewall behind the clutch master cylinder below the ID plate. The number was stamped AFTER the car was painted.
The wiper panel was changed around the end of 67 or early 68 with the change to the OZ type Lucas wiper motor. This was around the time the upholstery pattern was changed from the first type to the second type on Mk1 S cars. I used to think it was concurrent with the wiper change but I have seen one car with early type wiper panel and late type upholstery so its unclear as to when the two changes were introduced.
Also be aware that the Mini K car code was YG2S1 and there are some dodgy Mk 2 S cars that started life as Mini K's. It's pretty easy to overstamp the 1 with a 4 on the VIN plate to fake a Mk2 S. It's much harder to change the car number as there were only about 2500 Mk2 s cars built but many more Mini K's than this.
Hope this helps but do a search for more info.
Regards
RonR