Strange things can happen, though.
Besides my 69 RS coupe, the other car I have is a 66 Chevelle which has been in the family since it was first purchased on March 30,1966. I have all of the original documentation including protect-o-plate and have verified all of the numbers when the car was restored between 1995 and 2000. I even found a date code on the frame. Point is that everything is within a couple of weeks of the mid March 1966 build date at the Flint, MI assembly plant, except for the rear axle which is date coded Oct 1965. Axle date code, etc and POP totally agree. The rear end was build at the Buffalo Gear and Axle Plant and obviously set around somewhere before it made the trip to final assembly.
The issue of rear axle date code, what is correct, what isn't, what can be, what can't, etc. has come up multiple since this string back in 2005. I never got back to posting picutres which show what I had menntioned in the quote above (previous post in this thread).
The 1st picture is the axle code from my 69 RS. The car has a 10-Bollt, 2.73:1 open, mono-leaf rear end. Although I don't have any of the original documentation on my car, hard to believe that this isn't the original axle. The code is BP 10 18 G1; i.e. October 18, 1968. My RS is an 01C Norwood built car. 2nd and 3rd pictures show the ring gear in my rear end. 15 and 41 are the number of pinion and ring gear teeth; i.e. 41/15 = 2.733. The date code is the last two numbers; i.e. 9 68 (Sept. 1968).
As an addendum to the information that William provided in post #9 of this thread, and yes I know what I'm about to state is in reference to my '66 Chevelle and not the '69 RS, but it is still a Chevy product from the last half of the 1960's.
The 4th picture is the axle code from my '66 Chevelle. Picture is a little old and fuzzy, taken back in 1995 just after I had removed the rear end back when I was restoring the car. The axle code is CB 1027 B which is a 10 Bolt, 3.36:1 open rear end; October 27, 1965 date code. Although I don't have any original documentation on the Camaro, the Chevelle has been in my family simce my grandparents purchased it on March 30, 1966 from El Bauer Chevrolet in Decatur, IL I have all the original paperwork for the Chevelle including the POP which confirms the rear end code. The Chevelle was built at the Flint, MI plant in mid March 1966 (based on other date codes on the car including one on the frame). For whatever reason, the 3:36 rear end in the Chevelle was in stock at the Flint plant from early November 1965 until it went into my Chevelle in March 1966; some 4-1/2 months.
Based on the axle code on my Camaro, that rear end was in stock at Norwood from mid-October 1968 until mid January 1969; approximately 3 months.
Based on my own two observations as well as what William stated above, it looks like axle codes can have more of a time differential between axle code and car assembly date than what would normally be considered acceptable.
Just throwing this out for what it's worth.