According to Jerry MacNeish and his documented research, the D80 spoiler package became a mandatory Z28 option, with a price increase on April 1, 1969. According to my discussions with Larry Christenson, well known for his 1st Gen restorations and overall knowledge of them (he has had numerous unrestored 69's, plus his #1 of two actual 1969 Pace Car is pictured in Jerry's book, The Definitive 1969 Camaro Z28-SS396 Fact Book, on page 185) the tall "acorn" nuts were changed to Kelp nuts with integrated washer, approx. the first week of May 1969. Thus, it can be assumed that the switch was made about this time.
You can buy these along with many other GM NOS hardware from AMK Products (www.amkproducts.com
). The part number of this nut is 4821091 (w/sealer) or 4821092. I have an 05A 69 RS Z28 that had the original Kelp nuts on it and I replace with AMK NOS pieces. You can see pictures of the hardware on AMK Web site.