By chance is there an "R" on the cylinder face? Long shot, but didn't a slim number get sent back to the factory by owners with problems and replace these parts?

I have 2 66's that are no dash, prior to change. One an 8K serial number, just as it should be. The other a 4K serial with the gas ring moved etc. Back of cylinder face has a small "R" easily visible outside the ejector star.