What do these numbers/letters on my rear derailleur mean?

1st image is my rear derailleur, 2nd is the one I want to buy on eBay, 3rd is a wider image of the derailleur (which is the same for the eBay one and my one).

Just wondering if the eBay one will be compatible and what the 'A' and the other bit means. I have seen some without the 'A', would that be compatible?