So I’m trying to figure out whether I have a DI engine or a PC engine. The PC engines have glow plugs and the DI engine has an ether start valve. I do have a fitting on my air intake manifold that goes nowhere so I’m wondering if it had an ether start when you turn the switch to the left, my serial number is 79U3437 Currently, you have to use ether to start it. It will not start without the magic juice.
Your tractor was built in 1976 and should be PC. Photos of the top of the engine showing the fuel injector lines will soon show whether it's PC or DI. The precombustion chambers are quite obvious. Of course, there's every chance the engine has been swapped out for a different model of engine, lots can happen in a tractors life, in nearly 50 years.
Like OZ said take a picture so we can tell for sure