Apart from the info re differences others have pointed out there should be a production code, otherwise known as a Body CUM number-listed for Internal use only (No I don't know what CUM means but I doubt it has anything to do with sex), stamped on the body along with the other codes. It is most likely on the radiator shroud but could be on the firewall or the rear bonnet drain channel (Not 100% certain of the firewall or drain channel locations). The code is or should be
M2
06
This code was used for MK1,2 Cooper 'S' cars and possibly??? for 997 and 998 Cooper cars.
Clubman GT cars used
M2
23
and it was stamped on the firewall behind the Clutch master cylinder.
Some other Minis used
M2
05
So if you can find M2 above 06 stamped somewhere then it's probably a genuine car but you need to also check the other differences. If it's a 69 build car it won't have any ID plate unless it was built towards the end of 69. Cars built at the end of 69 on had ADR plates fixed to the firewall box section behind the brake booster. The ADR plates show the model code and car number YG2S4 ****.( Note that there is no K in the model code as this was deleted for MK2 cars. At least on cars with ADR plates) Any number above 2500 is suspect as there were only 2419? MK2 cars built. Note - CAR numbers start from 501 and body numbers from 001. The last 113 or so cars had the lift up door handles. Others have more accurate info re number of cars actually built. CAR numbers in the 1400 range were built in early 70.
The number stamped on the firewall mentioned by SKSSGN is the CAR number. The number stamped on the radiator surround is the BODY number. Car numbers are approx 500 less than body numbers.
Despite what Matt68 says the RH tank mounting bracket on the rear seat bulkhead is not welded but held on by 4 flat headed studs - not bolts.
Clubman GT cars or at least mine have the bracket welded.
The boot board support brackets are pop riveted in place. The floor pan should have provision for the full length skid plates. Other things to look for are 7 tab strips for the tank breathers - 2 on each side plus 3 under the rear parcel shelf. Extra holes in the boot floor for the fuel pump wiring and RH tank breather and the RH tank securing strap. RH tank support bracket spot welded to floor under tank. Tab on the underside of the boot floor above the fuel pump to secure the fuel pump breather pipe before it goes to the breather under the LH rear seat. Spot welded flair metal strips welded on the underside of the wheel arch openings - so they can rub at the front. The body and interior are basically Mini K but with the 130 MPH speedo. Instruments should be matt finish med/dark grey not gloss black as per MK1. Plastic bodied heater. Instrument panel with turn indicators as per Mini K. Mini K shorter steering column with dished wheel.
Hope this helps
RonR