Hi Bob,
Why don't you ask Henry? I always thought that the difference on the one piece Nelson plugs was the size of the hole/cavity were the wire coil sat. The other post were they talk about the notch in the Hex denote the HD seal If I recall. With the two piece globee type, Henry offered like 3 different filament diameters. Blue/Clear .007" Green .010" and Black .012 I think. Henry would be the best person to ask.
With the Italian TCA plugs (Blue Line), they offer different filament diameters, which give you a heat range. Smaller dia wire, 0,19 mm smallest, hotest to 0,29 mm coldest. You can get them in standard spring coil filament and flat coil (glowbee) type filaments.
My Company; B&L Hobby Supplies, Inc imports the TCA brand Glow Plugs. please contact me at williamhughes4@att.net if you are interested.
Bill