Peter,
You might ask around your region for the guitar seller or repair person that musicians agree has the most knowledge of vintage guitars. Identifying the exact year of manufacture, condition of the neck, small dings, replaced parts, repairs, or even well-done refinishing takes special skills. And any of these will affect the value.
You can look up in the Vintage Guitar Price Guide what the very best condition Mustang of that year would bring. I believe the value in the USA is now in the $800 to $1000 range, but please don't quote me on that.
Best of luck with your find.
