Storing info bytes in ATmega169PA that remain after reprogramming
If you're just talking about a bare chip, there isn't much point in retaining a unique ID if you're completely reprogramming the chip anyway. If you really want to do this, then putting a sticker on the top of the chip is probably the most direct approach.

However, if the chip is part of a larger assembly, and you want to keep track of specific assemblies, products like the DS2401 Silicon Serial Number are made for this purpose.