The chassis on a 109 is about 1" deeper section than an 88 so it will be easy to tell if it ever was a 109 chassis by the size of the chassis rails, It is still possible its been shortened but I doubt it. It is also possible that it was stamped wrong from day one, errors probably occured, but it also seems unlikely. My hunch is that it is an early 90 restamped to an S2A to get tax exemption but that it was done some time ago and its changed hands a few times since then. Its a case weighting up the probability of 2 options; 1 it is S2A that has the bulkhead, wings, suspension and axles fitted from a 90, or its a 90 that's had the reg plates, chassis stamp, and dash fitted from an S2A. Both have do occure but one is a lot less work. Can you post some chassis pics? Suspension mounts for example?