Slim,
They're not a Nasco accessory, but were made by Wilson Manufacturing. They're known as a "Wilsonia Guage", and there is a pic and information about them in our
Accessory section.
As far as value is concerned, they originally sold for a bit over 7 pounds, so I'll give you $20 for it. Sound fair? Didn't think you'd go for that. They're worth what someone will pay for them, but I've seen them go for $200. Depends upon condition, yada yada.
That "333" code means the tailgate glass was factory fitted, FWIW. Punch all the numbers into the
ID plate page and see what it tells you.
Hope that helps,
RET