Basically, Bob, I really doubt there is a difference. See my reply to your post in the main forum. I contacted a company called Aero Engines that sells and services Nelson engines and they show only one Nelson HD plug. the reply that I got from them and have posted posted in the other reply is the most I have been able to find out about them. Some guy is selling Nelson and "Italian Nelson" plugs on eBay and the ones he is calling "hot' have a notch on the hex like I was told by Aero Engines that denotes a HD plug. The Italian versions have a smooth hex and look like the old Standard Nelson plug. If you find out anything else, post it here, please.
  Type at you later,   
  Dan McEntee

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

The regular nelson plug has the hole just bigger than the coil and the other one (that works better with higher nitro and throttling) has a significantly larger hole. I will try to see if I have any to take a pic of tomorrow.
David