Hi Stuart,
I can't be much help on the question of the ID stamp as mine is the plain TC, however it wouldn't surprise me to find a difference across the production if it's something that's added by a bloke with a hammer on the assembly line ! To go any further I think you'll need some dismantling or possibly a compression test which most sellers might get wary about.
The +2 engine tale could be genuine but as has already been noted this would need the complete head assembly from the Europa's engine. It would also need the front cover as the outlet for the water pump is different (front rather than side outlet). It could be that the block was rebuilt (or cynically, just removed from the car) and the Europa parts swapped over.
So I think there's got to be an element of trust in the equation and personally I'd go on how the car looks & drives. Assuming the chassis/registration details check out and it's got the 5sp gearbox then from period road tests the Special should be virtually identical to the Elan Spring in terms of acceleration, so driving back to back might work ?
Brian