I came across this gem at an antique store the other day. I haven't been able to find the image on the internet after a couple days of light searching, but I did stumble across this forum and figured someone here may have some insight.
Any info or thoughts that may be out there are appreciated.
Sorry for the poor quality photo. Flash on in the dark...

Mike Slama would be a good person to contact on this company. You can google his name, he sells gas pump lenses and parts. Would be very helpful I am sure.

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:

Thank you so much for your reply and additional info!
Mine is definitely on illustrator's board,and I can see a few stray pencil marks here and there. Initially, I thought it might be an artist's proof of a newspaper ad, but my wife figures it is too big for that.
The framing is very nice, and it is covered by paper on the back, so I'm real hesitant to see if that tag you mention is there. Maybe I'll take it to our local frame shop to see if they'd be able to carefully check for me.
I may have to look into the artist and agency to see if I can narrow down a possible age - if I can figure out any additional info, I will post it here.
Thank you again!

Clack was in business from 1921 to 1954 based out of Havre MT. Husky oil bought him out in 1954. They have a Clack museum in Havre. Mitch

Here is a link to an article that tells a little about H.Earl Clack:
http://www.spokesman.com/stories/2016/aug/15/then-now-earl-clack-and-husky-oil/

Here is a tag from the back of one of the ads.

If you found that in Great Falls, it is owned by a member of Oldgas

Was owned by an old gas member perhaps...I bought it at an antique store last Wednesday, 12/19. I registered for this site and posted it this past Sunday.
The info that Husky Bob provided was extremely useful.

Finally found the other 3. They all measure about 10" X 20".

...if they are hand-painted, they are likely the original artwork used to make the billboard posters, and one-of-a-kind...they would have been photographed and enlarged considerably to create the posters, the sections of which would have been pasted on the billboards...

...exhibitions of poster artists' work were held, and the public could go and see the original paintings...the Outdoor Advertising Association held an annual competition for decades...they published a collection of the winners/honorable mentions for many years - starting as an annual issue of 'The Poster' in September, 1925, then as spiral-bound books...I have everyone from 1925-1960 except 1933 & 1939 😕...the exhibit of paintings were displayed in Chicago at Marshall Fields department store...

...N.C. Wyeth, George Petty, Haddon Sundblom, Leyendecker, and many others had their advertising work exhibited in this manner...

Those are absolutely amazing!

Do you have any idea on the approximate age of your pieces?