John Y
As you probably know Adam it is missing the pinstripe and the original factory alloy wheels. If you like originality, not that hard to source.
Not many photos or history provided to check and report more detail for you but a very nice looking looking car.
Russet brown was only offered in the 77 year last run of cars from what I remember and this car has the polished sill covers which again came in the later run of cars. With that VIN it does look like it was stamped as 5 cars off the final Stag VIN so this would have been completed around June 1977 and assembled on the 2000/2500 production line unlike earlier cars.
The main difference as you have noted being the lighter BW65 auto.
Some prefer the earlier cars and some like the colour/trim options of the later ones but unlike other classics the market has not given us any good reason for values being any higher for earlier or later cars - this of course can change. Like any classic the state of the body (inside and out) being one of the most important factors for 'value'.
The 'rego' is likely to be wrong here.
JY