What you have there is the predecessor of the Comp M2. That is the model you see mounted on the Delta M4's in the movie "Blackhawk Down". The original Comp models were in wide use in the military before the M2 was released a few years ago. There are actually a few of the older models still floating around judging from the military pics from Iraq.
The differences between the CompM and the CompM2 are that the M2 has a higher battery life, the M2 has a "crisper" dot, and a few more things that I can't think of at the moment.
The CompM is a great sight. The mount that is on the sight now is a $150 or so mount by itself. The other mount appears to be an Aimpoint factory ring and is not worth much.
Most of the CompM's I have seen for sale have been selling in the $250 range so you did VERY VERY well.
Raven, yes it is 30mm and it is a red dot sight.