Hi,
I have one of those IIBs modded to C+, too.
The amp has been checked by Mike Bendinelli in 8/2003 and Mesa-Boogie added a letter, which says that it is one of those few modded ones.
There were only 24 Bs converted to C+, because it is much more than a modding, it´s kind of "rebuilding" to do the mod.
They had to change the circuit board and that was only done a few times during the production period of the IIC+. Later on it was never offered again.
On my amp there is a mark on the chassis: "IIB to C+" , which you can find, when you pull out the chassis from the cab. And you can find MB and the + on the back. The front has been changed, too. So it says "Pull Deep". Ser.nr. and the day of birth (marked between the tubes) is fitting to the IIB aera.
Maybe Mesa-Boogie can tell you more, when you tell them about the ser.nr. or send the amp for a check.
Pics from the inside would be helpfull, too.
BoogieBabys is very knowledgeing, maybe he can see, if it´s a C+, now.
I like my modded IIB very much. It is the best amp I ever owned.
I have another C+, which was born as a C+ and I like the modded IIB more. They both are fully loaded. I also owned two other C+s, which were a 100/60 W, RV and a 60 W, RV, no EQ. Those have been superb amps, too. But I sold them, to buy another fully loaded one, which I´m waiting for at the moment. I hope to have it next week.
From all the C+s I owned/heard, the modded IIB is the one I like most.
Maybe you are one of the lucky owners of those very rare and great amps.