Deere used to sell the window under a Deere part # that has been obsolete since the 1980s. The Stanadyne part # for the round plastic timing window for C pumps is # 17180. Thompson diesel was selling them for $8 each but I suspect they are all gone. I bought a spare last year from them. Other dealers charge around $25 for it.
The round plastic window has degree lines in circles. You screw it onto the pump and then start the engine. Rev it up and look into the window to see if the timing changes. Very easy to do and with Deere spec pumps - you can adjust the timing advance with the engine running.