Got down to preparing the rockers for paint. I decided to take them all the way to metal due to the heavy rock chips. I found this stamping (very deep) on the underside. Must be a part number and 65L almost certainly refers to 1965 Left Side. I did not know these were stamped. And yes, I am absolutely overjoyed to have found the outer rockers in this condition. The inners are fine...still have nice glossy factory paint on them.

 

Month, date, shift, stamping plant. The repops don't have this.
http://www.hammar.dyndns.org/datecode.htm

 

That means that its a 65 left side, made on december 18, Chicago Il, 3rd shift.