At one point I had both Mk.III 105 and C+ 105 and the C+ is it's own animal. The Mk.III is smoother and much more modern sounding with a compressed feel when heads up to the C+. The C+ is like a chainsaw or dirt-bike, where as the Mk.III is like a race car.
With the right guitar, you could get either to sound very similar because a guitar will change everything.
Looks like a black dot 1x12 that someone hacked down to a head format and re-covered. I personally own a Mike B upgraded IIC+ SRG and III+ DRG red stripe with the R2 mod; the tone is 99% identical on the lead channel. I call placebo effect. Lol
That cabinet is a custom creation and I'd bet money that it was not made or covered by Mesa. Even the style of the covering is different, and I'm sure I've never seen that tolex pattern on even a custom ordered Mesa amp.
75 or 85 watts, basically a twin of my black dot simulclass Mark III with reverb and EQ. My serial number is in the 15300 range and mine has the export transformer. But no + mark in the upper corner and I'd like to know for certain what that means.