Well gents and ladies, I've changed my opinion about these odd-ball heat shields - they are removable. Thank you Screwdriva for creating this thread and asking the question. It forced me to take a closer look at one of the pre-chambers I have in my stash, and to ask fresh questions myself.
1 - Is the heat shield riveted in from below? Take a look at this:
The above photo is looking into the pre-chamber through the hole the glow plug tip passes through. You can clearly see the bottom of the heat shield protruding through the hole. Also, it does not appear to be peened over, or riveted in.
2 - If it's not riveted in, how easily removed is it? I used an appropriately sized drill bit inserted through the glow plug hole to push the heat shield out. I levered the drill bit against the edge of the glow plug hole and with a couple pushes the heat shield came out relatively easily. Here's what I ended up with:
And the heat shield itself (the metal is much softer than that used in the pre-chamber):
3 - Can the "normal" heat shields be used in these pre-chambers? I am less sure of this but I suspect the answer is "yes", provided the angle at the bottom of the chamber, where the heat shield seals against the pre-chamber, is the same. FWIW a "normal" heat shield drops right into place. I took a photo of that too but I am limited to 5 images per post so I'll leave it out.
So, is this heat shield the so-called "top hat" style? Sure looks like it could be. I'm going to dig out the books and see if I can find any information about this question.
Another question is how to remove these heat shields from above without having the pre-chambers out of the head. I think hercules' approach might be the thing to try (easy-out). If you can get a tool in there that grips and turns the heat shield I bet it will come out relatively easily. My previous attempts were hooking, pulling and prying and I had no luck.
Side note:
Something interesting I noticed while looking at this pre-chamber is that the top and bottom parts of it are threaded together. You can see this in the first photo I posted in relation to question #1. The lower part of the pre-chamber (top in the image) is made from a brighter alloy, and the upper part of the pre-chamber (bottom in image) is made from a darker alloy. Look at the exposed cross section shown in the edge of the glow plug hole and you can see the threads. Other types in my collection of pre-chambers are the same.