I have a S373D, Beaver III. I also have the original worn Instruction Book.
This is from the Instruction Book, page 6, step 3 from section on how to start.
"3. Insert the starter key into the key switch and turn it counterclock-
wise and hold it at "Heat" position to heat the glow plugs. As soon
as the glow signal on the instrument panle glows red, about 30 seconds
later rapidly turn the key to the "Start" position and the starter
motor will rotate and the engine will start.
4. When the engine is started, take your hand off the key and the key
then automatically returns to the "ON" position. The key must be
in the "ON" position while the engine is running. "
On mine the glow plugs are only warm while I am holding the key in the HEAT position.
I seldom use the glow plugs since I live in South Carolina.
Let me know if you want any more info from the "Instruction Book" that I have.