I think the same thing is true for all the Rallye wheels of the late '60's at least. Gray paint was applied by Kelsey for 'service wheels', destined to be sold thru the dealerships, rather than production line parts.
What I meant to say is ALL (5) of my ORIG factory (LOS) installed 76kmi rallys wheel are KH gray on the back side. There is NO black. I have owned the car since 83. I am convinced my car was purchased with the similarly dated orig factory wheels.
FWIW Colvin Book FIG 17-8 1969 14x7 jj standard wheel Kelsey Hays drawing number 082153 seems to confirm that wheels were part numbered with varying finishes ,,,plain, rustproof oiled,,, primed KH spec 3323 black..,,,,