I'm viewing some local listings and ran into a guy selling a 1981 Mark IIC or IIB. He isn't sure because he doesn't know what the difference is. The thing I'm dying to know is: Can the C/B be upgraded to the IIC+ still? If so, how much vaguely is it to do this? Do I need to look for any specific serial numbers to make sure what it is? I'm vaguely familiar with the IIC+, and if this can be converted, then it's a steal. Help would be greatly appreciated.