series 3 has a full synchro box whereas the 2a only has synchro on 3rd and 4th, 1969 2a should have headlights in the wings i believe, along with guages in the centre of the dash, heater matrix inside the cab, a single wiper motor mounted to the passenger side of the bulkhead, dipswitch on the floor and possibly a wire spoked steering wheel with a horn button in the centre...
series 3 has lights in wings and an air intake hole for the heater in the nearside wing, "snail shell" blower and heater matrix mounted under the bonnet on top of the passenger footwell, guages in a plastic facia in front of the driver, three spoke steering wheel and a combination dip/main/indicator/horn switch on the column plus a more "refined" dash board that is plastic clad ...
these are the main interior differences that i know of, externally the door hinges differ (smooth on the S3 and proud on the S2a), screen hinges differ as well so the screens aren't interchangeable either (earlier S2a has yet a different screen that carries 2 wiper motors as per my 1965 109 2a)...
standard seating in both is pretty basic unless someone has fitted deluxe seats (still basic)
there are various small differences that i've heard of that the more knowledgeable would be able to point out but i doubt i'd spot (only been into Series landrovers for just over a year) but i hope that helps you with your decisions
all these identifying points assume that no one has swapped parts over the years for bits they could get cheaply to keep thier motor on the road and usable (my 88" is a proper Bitsa)