IĞm uncertain how/where to read the revision

I have some DIL40 marked

16F877-20/P
9924BSL

and some PLCC

PIC16F877-04/L
9925SBN


First I thought the first one is rev B  ("BSL" after date code)
But then I see on the PLCC "SBN" after date code.
Still a B involved, but in another position...
Looks like S revision...

Strange!

Or is the characters after the date just manufacturing lot etc, and theese are not 'A* nor B revisions, but bug free (supposedlu) ones?

Thanks in advance
/Morgan