Bumper Datecodes

Does anyone know for sure how to decode dates on bumpers?

I've found 2 fronts with the following codes:

1 21 M6

3 3 M5

Using the sheet metal codes they would be

1st week 21st day Monroe, Mi stamping 6th shift (or year???)

3rd week 3rd Monroe, Mi stamping 5th shift (or year???)

I haven't yet found any rear bumpers with clear codes yet.

Any help is appreciated.

FYI - I'm looking for a good front and rear core for re-chrome for my April '65 K. If anyone here has some for sale please let me know.

Thanks - just wanted to confirm.

Can you (or anyone) confirm if the last number is the year or is it part of the factory code (shift)?