Occasionally, parts can be a little closer than 30 days prior to the build date. December 1968 was a 5 production week month, meaning the 3rd week of January 1969 is a little later than you realize. My 01C X77 '69 Z had the MSO cut on the 28th of January (the date of release of the car from production). I have seen several quotes that suggest the actual build was on the day the MSO was completed and the car assigned/shipped to the dealership. I have parts as close as 10 days, and as far out as a couple of months. Other original Camaros I have bear up to the same relative time periods for parts as well.
Remember, these are just my observations - I don't pass myself off as an expert, just a long -term Camaro owner/enthusiast (39 years).