As already mentioned their all the same drive just different naming variations though the Pro is a 1620 with B7P9 firmware because of the new features in the firmware BenQ gave the drive a new name to market the features and is phasing out the "1620" name.
As for the â€œAâ€ I thinks thatâ€™s just part of the full name, everyone calls the drive the 1620 but the full name is the BenQ DW-1620A
OEM means Original Equipment Manufacturer its when companies make a product that is intended to be sold to another company which will it turn sell it on as part of a system etc.
Lots of OEM stuff turns up in retail though so basically in most cases itâ€™s just the drive on its own nothing else.
The retail package comes in a box with software (Nero), a BenQ 16x DVD+R disc and a standard IDE cable plus some screws etc.
One final difference is the BenQ put different firmware in their OEM drives but you can cross flash to the retail firmware because the retail firmware tends to be update first with the OEM later.