Help..Differences between CAT SL-1 Mk II & Mk III

I would like some help with the differences (internally) between these two models since they have been upgraded several times over the years.

CAT SL-1 Signature Mk 2 and CAT SL-1 Signature Mk 3

I'm almost certain that I currently have a Mk 2 but would like to be sure. I've tried contacting CAT with no luck so all you CAT owners out is needed!

Thanks so much...
Hi, take a look at the tube type no's, the MKIII was modified to use couple of type 6922 tubes. Also with the serial # CAT should be able to tell you the history of the unit.