"There have been documented +’s as low as 12337, mainly in some prototype, nearly stock or customized circuits. In recent months I have witnessed two IIC I’s. These are IIC’s that almost made it to a plus, but had “Pull Deep” faceplates and a stock IIC circuit with the RP11A circuit board. These were very interesting times at Mesa. A customer could have chosen a C or a C+, hence, the “I” looks like the initial swipe of a +, but never got the other horizontal swipe so do your loop tests. I could imagine Mesa having to call on all outstanding IIC orders and asking if they wanted the upgrade. In the C+ transistion around 12,3XX/124XX"
I bought 12310. It's a +, passes the loop test etc, says +. I bought it from the original owner who's had it since the mid 80s. He bought it new this way, he never sent it back for modding. I guess that makes it a very early factory upgraded IIC.
