H77 (PN ending 192) = 2009 unit and anything before. It has the standard res screen and only accepts SD cards. Not 100% sure on the limits but I used x2 4GB ones in mine when it was installed.
The 2010 model (PN ending 193) = Am not sure of the HW version as I forgot to note mine down before I installed it but it has a Higher res screen than before, SDHC card use (16GB x2 I believe), faster processor, redesigned user interface, Audi AMI (CAN based). Am sure there is more.
I would imagine that H numbers are changed during the production of these units in order to note a change in a component, internal parts supplier, visual item (chrome dials), factory shipped firmware maybe. TBH my H77 seemed no different to any other older RNS-e bar the chrome dials.