AB, my 312 was on the hopper as you can see from my first pic that I posted. I looked at the crossbar that your 312 is on, and there is nothing there unless it it under paint, rust, and crud. 

Yours might be a later model 312. From what I have come to understand there was only 2 years a 3 point planter was made, 1962 and 1972 or 1974 I not quite sure on the last year, According to Miabach Tractor.