Per this article http://support.apple.com/kb/TA21608, the model number you'd want is M4551A. I believe the only difference, other than the plastics, between M4551A and M4551B is that the former has the older "Mac" style video input and the latter has VGA input.

The later M7613 (graphite/DVI) model adds a USB hub, removes the ADB hub and I think it removes the analog video input of the two M4551 models.