Note, Tommy, the 2a had headlamps in the wings from the start of 1969 until the S3 came in in 1971; your abandoned "1969" pickup could be genuine with either headlamp setup.
Judging by what I see on ebay and so on, there seem to be a lot of S3s masquerading as (deceased/scrapped) S2s, no doubt with the aim of getting tax free status, so the documents and the chassis number on the bulkhead plate don't necessarily tell the true story.
Easy to check tell-tales are:-
Bulkhead: the S2a bulkhead is very different from the one on the S3, the most obvious features being: the S2a has a metal instrument panel mounted in the middle; the windscreen hinge brackets are flat plates welded to the outer edge of the bulkhead (and similar flat plates welded to the outer edge of the screen frame); the door hinges protrude similar to those fitted to the rear safari doors, they're not "flat" like the S3's.
Heater: in the S2a it'll be a recirculator fitted in the cab under the instrument panel; in 1969 it would almost certainly be the square Smiths type. The S3 has a much more powerful "fresh air" heater with the working parts mounted inside the front wing above the passenger footwell, with an air intake on the side of the wing.
Rear axle: the 2a 109 will have a Rover rear axle on which the diff unit is removed by taking it out of the FRONT of the axle casing. The S3 109 has a Salisbury axle on which the diff unit is removed through an opening in the REAR of the casing so it has a very obvious bolted-on diff cover plate at back.
Fuel tank: the 2a 109 will have the fuel tank under the driver's seat (except station wagons and .... maybe 6 cylinder versions?) with the filler in the same place as on the 88, i.e., next to the driver's door. This carried over to the first year or so of the S3 109 after which the tank was moved to the centre rear of the chassis with a filler behind the o/s rear wheel arch. Ex military S2 to late S3 109s mostly have the tank under the driver's seat with the filler cap directly on top of the tank.
Grille: the headlamp-in-wings 2a should have a galvanised steel mesh "maltese cross" grille but since fitting a plastic S3 grille was a cheap and quick "improvement", most of them lost the originals early on. To my shame, I "modernised" my 1970 grille circa 1978 but fortunately I kept the bits.
Gearbox: the 2a has no synchro on 1st and 2nd (the S3 is all-synchromesh). I feel the "throw" of the lever is considerably more between 1st and 2nd than in the notchy S3 box. If original, the gearlever will be noticeably cranked on the 2a, possibly with a mushroom shaped knob but this changed to spherical like the S3 around late 69. The straight-cut first gear on the 2a (helical cut on S3) will give its characteristic noise if you know what to listen for.
Obviously things like gearboxes and axles get changed but I'd be especially wary of a S2a wearing a S3 bulkhead unless there were sufficient other factors there to convince me it was the vehicle it was claiming to be and that the bulkhead was a bona fide replacement.
