Based on the date, that would put it into Mark IIC territory. Based on the "+" over the power cord, that would signify that this amp is a Mark IIC+ Long Head based on the pics, but gu shots (close up pictures of the circuit boards and components) would help to better ID your amp.
If you want to make sure this actually is a "+" and not someone just trying to pass off a Mark IIC as a Mark IIC+, you need to perform the loop test.
That's a nice amp. Just run the Mesa GEQ pedal in the loop or send it into to Mike B to add the GEQ (though they may not have any more of those face plates).
But IMHO those things sound great without using the GEQ. It's kinda fun for a while to pump up the bass, but whenever I play a Mark for an extended period of time, I generally end up bypassing the GEQ anyway.