This was from the era where WMS was experimenting with Diamondplate.
10 Banzai Runs and 10 Earthshakers had the special 'XR7' coating, later called Diamondplate.
I've been told 200 Whirlwinds and 100 Rollergames had Diamondplate.
As many of each as I've seen, I'd suggest the figure was more like 1000 Rollergames and maybe 500 Whirlwinds. There were 100 or so Diner machines with DP, too.
The games that are uber rare with Diamondplate are Radical! (I had one, foolishly sold it to buy another pin that wasn't as good as I thought it was!), Pool Sharks and Riverboat Gambler.
I have heard that some Bugs Bunny games had DP, but I've never seen one of those. I've personally seen or owned at least 1 of each of the other titles.
Since Diamondplate wasn't yet proven, operators who had used mylar for years still wanted it on their games. A factory mylar was made for the games, so they used them.
They also did DP on about 200 Funhouses and 100-200 Bride of Pinbot machines. They also tested a coating called L.S. Top Coat on some Funhouses. It didn't hold up as well as DP did. I had a topcoat Funhouse, and while the playfield colors were still as vibrant as new, the finish was cracked everywhere!
Aeneas has some great info here: