So is the 11B on the trim tag a Fisher date and the official production date a GM date?
Yes, 11B is the week of Nov (2nd week) that Fisher anticipated starting the body assembly. For whatever reason, whether it was bad information, delay, order volume or other issues, your car was started much later. My guess would be more like 11D. The production timeframe was about four days or seven shifts. That information is found in this excellent report by JohnZ;
http://www.camaros.org/assemblyprocess.shtmlAs I understand it, the production date indicated on your NCRS letter should be the date that Chevrolet reported that final assembly was complete. The attached article link covers the entire process; I think it is a must read if you are at all interested in how the processes worked.