I’ve never seen a Dragon with a 1/2” thread on it, to be honest I thought they used the m10x1.5 because the barrel was too thin to take a 1/2”UNF thread. I do have an M10x1.5 to 1/2” adapter made from brass that came with mine, but couldn’t say if it’s an official part.

As an aside all the Parker Hale silencers I’ve seen with the protrusion to allow use with Dragons have all been m10x1.5, but then again I’ve only seen a handful.

The H on the barrel I’m sure signifies something they did to the bore of some of the barrels, but for the life of me can’t remember at the moment

My Dragon FT has no thread at all, just a plain muzzle to the usual threaded length… could be that they just used the barrels they knew they could machine to fit.

I would imagine that Keith (justinthyme on here) may know more on the subject