[C320-list] Ignition Switch and Glow Plugs (Perkins M30 Engine)

[Dave Hupe]

 I currently have as 3 position engine keyswitch and no "start" button.  The keyswitch positions are off, on, and start (spring loaded to return to "on"). I supposedly also have glow plugs (I know this only based on the previous owner saying they had ben replaced). 

   I don't think this keyswitch is original.  My Perkins Perama M30 manual shows a keyswitch with 5 positions (P, O, I, II, III).  The manual talks about turning to position II for 20-30 seconds and heating the glow plugs, then turning to position III to crank the engine (although the diagram in the manual for position III says both "glow plug and crank").
Based on reading some other posts, am I correct in assuming that when I turn the key of my 3-position switch to my "start" position that I am simultaneously heating the glow plugs and cranking/starting?  Or, when I turn to the "on" position, is that to first heat the glow plugs, and is the "start" position only to crank the starter?  
Dave Hupe
1994 Cat 320 (Hull #32)
Holland, MI