There are light assemblies on Oklahoma's turnpikes at the tollbooths and at entry and exit ramps (Creek/Muskogee interchange, for an example.) The lights on the Creek Turnpike light up "Thank You" while the lights on the Turner and Muskogee Turnpikes say "Pikepass Thanks You." Which style came first? When were the lights on the Muskogee and Turner Turnpikes installed?
Can't say when the lights specifically were installed, but the current tolling configuration on the Turner (the pay at mainline, refund at sidegate method) was established on January 1, 1990, and the Creek wasn't even open to traffic until 1992.
Have you noticed any variations at different Creek plazas? The road opened in three phases (check out the Wikipedia article, which is currently standing as a Featured Article candidate), so if you notice differences on different segments of the Creek, you would be able to narrow it down significantly, barring any light replacement since then.