Having working in manufacturing operations my entire career I know that dates "on paper" do not always correspond to what happens on the shop floor. For some reason the RS lens became a problem for Chevy and the decision was made to discontinue them as of January 1969. At that time there may have been some inventory at the source, in transit or at the plants. Someone has to decide the disposition of that inventory. It may be scrapped, shipped to GMPD to be used as service parts, or simply used in production until exhausted. So it is possible some RS cars were built with them into 1969 and others may have acquired them in repairing collision damage. They were serviced for several years; NOS still turns up.
In general an RS built after January 2, 1969 would not have them.