This is the options list. X-codes are on the trim tag under the hood and I beleive they were on Norwood built cars only and only for a certain amount of time. I am sure more people will have more details.....
69 SSRS 396(roller 427 ) My Chevs
Yup, they are on Norwood cars built after about Jan 1, 1969. They decode as follows:
X11 Style trim Group.Basically, extra Chrome ---Could be any non-performance engine, Could be RS, but not necesssarily, Could be RS/SS350,Could be SS350
X22 SS396 with Style Trim Group, or RS/SS396
X33 Z/28 or RS Z/28
X44 Base Trim could be any non-performance engine Also, most COPO's were X44