When I decided to make the 2 types of Pencile Glow Plug type reamers that I make I bought 1 each of the Factory Made ones. However the Loop type one cost too much and the demand is low.
Most people seem to want to switch to the up-date Pencil Glow Plug kit due to the better performance of the kit.

I also think since the Loop style GP hole is big you could easily us something like a 20 Gauge Shot Gun Cleaning Brush; which come in Bronz or Stainless Steel (careful not to buy the Nylon one that is not stiff enough.)
However, it would be handier to also have a short section of the Shot Gun Cleaning Rod to mount the brush on.
It may even be that dipping the Brush in Gun Cleaning Solovent might help clean the Carbon out too.
But I have no example of an Engine with the Loop style plugs to try this out on so someone who owns one will have to do so.
If you want to try it measure or take the Glow Plug with you; I believe even Wal Mart sells the cleaning brushes.

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84 300D, 82 Volvo 244Gl Diesel