Great sign, and nice score Tony.

Tony can you confirm the actual length & height again?
Based on that photo,doesn't look 98" long X 22" tall.

Rick, it is 98.5" x 22".

Interesting vintage sign,I don't think it's something that someone made up recently at all.It looks like an authentic Chevrolet auto show piece,judging by the series plug connections and the box & lettering design,probably mid to late 1970s.
I have many vintage Chevrolet lighted sign catalogs,and don't see that design in any of them.It might have been part of a generic truck show display package used at the Bottlers' Convention,or other similar trade shows.
I also looked through my limited amount of 1960'-70's-80's auto show photos and didn't see anything like it among the other sign lights.
Very cool!

Rick,

I thought the GM specification called for a ribbed face for durability

Pretty much standard for brand signage and OK signs. Comments?

Campbell Ewald was the vendor supplier to GM for these signs.

Companies taped by GM all had their own design ideas,so there really wasn't one specific template.Some outdoor Chevrolet signs in the early to mid 1960s were ribbed,but not all were as ribbing wasn't a requirement by anyone.
Dealer showroom signage,both outdoor and indoor,was handled by GM Photographic.
Auto show signs were designed and made by the Chevrolet Show & Display group,for which outdoor durability wasn't usually a design factor.All signs were of course,subject to Chevrolet Sales Dept. approval.They had the final say.
Hanover Mfr. was also a sign company that GM used to make their indoor signs,clocks etc.Campbell-Ewald wasn't involved with any lighted signs,they only got involved with lengthy advertising copy such as descriptive paragraphs on displays that needed to be written.

Rick.

What I am saying is that as the vendor supplier to Chevrolet- Campbell Ewald managed the advertising campaign and the design specifications for GM logo specification to the subcontract manufacturers,- one of which was Hanover for signs. There were other vendors as well including trade show display subs.

It was my understanding that Chevy Show and display worked in close coordination with Campbell. Is this incorrect?

As one example even pace car decals were sourced for production through Campbell Ewald by Chevrolet.