Bullitt's 0815, without the C (but with the 341 rear), and Sarge's WITH the C, stamped 0820.. pretty well brackets the change to use of the 'C' (1970 format) to between the dates of 15 August and 20August 1969. Bullitt's diff is the later casting number, but without the 'C', so that seems to imply that the two changes (C) and 341 casting, did not occurr simultaneously...
The rear end casting, and use of the 'C' in front of the diff app stamp are two more items to add to our list of intended 1970 changes that were applied to 'late 1969 extended production of Camaros and Corvettes. I think it would be a good research report to itemize all the things that 'changed', as well as best known dates of change, for late production, and would be useful in the generation of an 'ultimate' Restoration and judging reference manual for Camaros.