The latest DIR-655 hardware version is A4. I just bought a new one from newegg.com on sale with their discount code and they sent me a version A4 with firmware version 1.20 installed on it.
There seems to be DIR-655 A4's that come with firmware V1.20 or V1.21 new out of the box.
Beware that if you buy from places like ebay or used, that you probably won't get the latest version A4 unless you verify it with the seller! You probably will get hardware A1 or A2. You might get an A3 too, which some folks seem to prefer. I prefer the A4 as it has the latest and greatest Atheros chipsets built onto the motherboard (as opposed to older A1, A2 and A3 design which used a separate mini-PCI adapter for the radio card). My take is that there is less to go wrong (no connector contacts to corrode and go intermittent with time) on the A4. But you cannot modify or change out the radio card in the A4, you can in the earlier versions. A3 and A4 seem to be the way to go with firmware V1.2x. DO NOT upgrade the firmware to latest V1.31 at this time as it is wrought with folks complaining about instability and bugs! nuff' said.