I just now measured my Skeets set rings circa 2002 with a set of telescoping gauges and micrometer from LS Starrett.
While they do not have a high polish, they both measure within .001 of target which is about as close as I can get given a little waviness in the set rings. Close enough.
I never thought to check before now. I always knew that waviness was there, at least in the .700 ring.
I just talked to Ed Patten and says the 'O' ring on the older two head units is 3/8" ID, 1/4" OD, 1/16" thick. A common size soft rubber ring. My 'O' ring has turned hard and provides inadequate friction. Apparently, the current units are single head and do not require the O ring.
