[Mr70]

That Chevrolet advertising design first came out in 1970,not before.
They continued with it well into the next three decades.
Try to post a close up pic of the bowtie here,as it may be able to tell the decade.
It's far away,but looks more modern to me then 1970.
Try looking on the bottom border for a date code resembling something like CE 77 for the maker/year.That being window trim,it might be double sided.Blue is usually unstable from the get go,so there probably showing heavy sun fade more on one side then the other.
Neat piece...any illuminated signs inside?