Looks like some that I have.
I'd say around 18 years ago a friend of mine told me about an ad he saw in the Spokane Newspaper for 6 signs very similar to yours.
I called right away, but was too late as the guy had already sold them for $50 each. He said he worked in a building that used to do advertising and there was a big clean up and he found them in the dumpster, so he pulled them out and advertised them for sale.
To me they looked like artists renditions of Hi-Way billboards. Turns out I knew the guy (Jimy the Weasel) who bought 5 of the six and was able to buy them from him for considerably more $$. He said the one he wasn't able to get had a buffalo on it.
Each sign that I got had a tag taped to the back with these names typed on it:
Advertising and Sales Association of Spokane
Artist John Segesman
Agency Hayward Larkin Inc.
Client H. Earl Clack Company
They were all hand painted on Illustrators Board. You can even see where the artist had made changes in some of them.
A number of years ago Dwaine Buck made some copies of them with a decal on powder coated metal and they actually looked pretty good.
Hope this helps you out. Here are a couple of mine: