The “A” is the edition without a barcode. “AP” is polybagged with no barcode, “AU” is bagged, no barcode
The “B” for this issue is the version with a barcode with the “P” and “A” referring to bagged and unbagged respectively.
Yes, the “1Silver” is a different colored version of the cover.
For the Marvel Zombies, the letter refers to the different printing and different cover variant. The letters don’t always mean the same thing from comic to comic, but are assigned sequentially to differentiate between different covers that may share the same issue number or are exclusives made for Dynamic Forces, NY ComicCon, etc. An example to look at is Archie(2015). The first issue has 32 different covers and there’s only 26 letters in the alphabet, so there’s further descriptors on several books(there’s a “1B” and a “1BAM” for instance, and neither was polybagged).
Hope I didn’t confuse things. It’s just different ways to denote multiple versions of the same issue number.