If someone is thinking of buying this and you are worried about it's authenticity, you should call Mike Bendinelli at Boogie and ask him if the amp's serial number can be verified.
MesieBooga... you should really post pictures of the back as well as the front for serial number verification. That's pretty penny to ask on faith alone.
The boost indicates a IIC but it also indicates, as you say, a very early IIC+.
http://homepage.mac.com/mesaboogie/IICPlus.html
Quoted from The Boogie Files (Thanks Ian):
Mark IIC & IIC+:
The Mark IIC+ was the last of the Mark II series and featured a more sensitive (i.e. useful) lead channel and, more importantly, an improved circuitry in the effects loop. Unlike earlier Mark II models, pedals could be used without the amp's signal overloading their inputs. You can tell if a particular amp is a "+" by looking for a hand-written black "+" mark directly above where the power cord attaches to the back of the amp. Be aware that many people are want to jack up the price on a Mark IIC+ but often don't know anything about what the "+" means . Indeed, the mark itself can be forged, so let the buyer beware. But, you can figure out if it is a true IIC+.
Knobs:
The knobs on the Mark I through to the Mark IIB are the silver center ones similar to Fender amps. With the introduction of the Mark IIC MESA switched to the all plastic knobs with the flat tops, with white inlaid numbers, used in the Mark IIC, IIC+ and the Mark III up to the '90's. The knobs remained the same shape and color, except in the '90's the numbers were not inlaid and were of a cream color.
Front Panel:
On the Mark IIB there was no Pull shift on the Bass, whereas the IIC and IIC+ had this feature. Both the IIB and the IIC had Pull Gain Boost with the Master 1 Volume, the IIC+ (save for the first few or IIC's that had been modified to IIC+) had Pull Deep function with Master 1.
IIC+ Tone Controls:
The two input jacks on the front panel are marked Input" and "Foot Switch. The lead/rhythm footswitch plugs in and has a red LED to indicate function in the rhythm mode. The front panels read Volume 1, Treble, Bass, Middle, Master 1, Lead Drive and Lead Master. It has "Pull-Bright" on Volume 1, "Pull-Shift" on the Treble, "Pull-Deep" on the Master 1 and "Pull-Bright" on the Lead Master"
Date of Manufacture:
Like all Mark amps, the Mark IIC+ amps have the date of manufacture written on the bottom of the amp. For a IIC+ this should be very late 1983 to all of 1984, and very early 1985.
Markings:
A true Mark IIC+ will have a "+" over the power cord and Mike Bendinelli's initials on the back below the cord and on the bottom of the amp. As well, on the side of the amp chassis will be the MESA amp code DRG. As well, the date of the amp's manufacture will be written on the bottom.
Serial Number:
To further validate the IIC+ claim, look for the serial number. It should be between 125XX and 14XXX, but there is variability with the low end of this range as the exact start of the IIC+ and end of the IIC was in the 12,3XX range.
Mark IIC+ Effects Loop:
To have the definitive proof that an amp is in fact a IIC+ claim, one must first ascertain the above, but to prove beyond a doubt, there is a simple effects loop play test.
1) Plug your guitar into the fx return with the amp in LEAD mode.
2) Play a chord and let it ring out.
3) While it is ringing out, adjust the lead drive pot.
If there is a change in sound as you adjust that lead drive control - it is NOT a C+, it must be a Mark IIC. Reason is because the lead voice was placed entirely in the pre amp section in the C+ whereas the original C had some shaping after the pre amp which is why that lead drive would effect the sound when you plug your guitar into the fx return (bypassing the pre amp section).