The thread on the PPK should be metric fine… interestingly if you look at the commercially available adaptors online the suppressor thread is imperial. 
But then, that is to fit a US made silencer.
I was looking at that myself.
From what I can find out that is for the PPK/S .22 which comes externally threaded M8 from the factory. That particular one adapting to the more common 1/2x28 for the US as you mentioned.
3/8 BSF would have been my guess if this was something US manufactured as well, but as I’ve found for quite a few years now, the use of Imperial in manufacturing and parts is slowly being wiped out by Metric.
Considering everything is made in China across the world now, whatever China decides is what we are stuck dealing with!
It’s getting very difficult in my industry simply to find quality aftermarket Imperial Automotive bolts, which used to be readily available everywhere, but now can only be sourced from specialty Fastener Suppliers 
It’s also created a whole new level of what I call “Chinasize” nuts/bolts/threads, which is when working on a machine that has sizes that don’t match anything ever known to mankind 
Even in the massive manufacturing/machining/fabrication Workshop where I worked during my years in the Mining industry, we were retrofitting/modifying/machining all of the massive refining equipment from Imperial to Metric as each machine came through for rebuild repair……… where we would convert every single nut/bolt/internal threads over to Metric sizes, because even back then they were finding it difficult to locate the millions of different types/sizes/threads and tensile type bolts we needed in massive quantities each year.
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Yes. Similar experience here.
I had three toolkits I had to cart around to breakdowns. 1 Imperial. 2 Metric. 3 fuck you china inbetween half assed shit I’ll drill, oxy cut you out and either make you metric or imperial.
Such a head fuck, but as we had modern machinery with things like Festo pneumatic which was easy to work with and all metric.
But we also had old WW2 British presses. Then there was the inbetween shit you just gut it and make it your flavour which we just always went with metric except for the english presses but that was fine as parts were manufactured for them locally from scratch as nothing was available. ABS brakes re did the pads for the 12ft diameter clutch and brakes. It was pretty full on. Good experience. Cranks we had to get scratch built and replaced a few of those with a 10t mobile crane. They were anything from 10" up to the big boy 10 times the diameter.
They did the job and made millions of car wheels for 60 years for Australian made cars. Also some prominent overseas cars like Toyota Camry and Mitsubishi.
@DocBob how do you like UNC, UNF. Then the pipe threads BSPT. Then the Brit threads BSW, BSF. 
Had all of them. Then throw anything that was Japnese in to the mix which is made to their JIS standard… just because 
Some days you’d wear out a track back and forth to the on site $1.6M maintenance store just trying trying to figure out what fucking bolt or nut you needed 
Actually the JIS standard is good they stick to the metric thread size, just mostly the what we call phillips head is different and JIS really need the JIS screw drivers. Found in most electrical gear and anything made in Japan to this day, Toyotas, Yamaha, Kawasaki Suzuki Honda, Mitsubishi. Was part of the Japs industrial revolution after WW2. Japanese Industiral Standard.
Speaking of weird Chinese threads, check out the 14mm CCW thread on the end of the “outer barrel” of my LeHui SIG 552.
I use the term outer barrel very loosely, it’s not a full diameter outer, more like a half barrel. The only piece of it that’s full is what you see sticking out the front past the fake gas block. The rest of it is more like a half channel for the inner barrel to rest on.
So we’re talking a diecast 14mm CCW thread… with no way to easily change it to a quality outer barrel with a solid 14mm CCW thread because of the proprietry design.
There’s only about 70% of full thread, the threads are worn and rounded off already and about the only thing that fits it without being loose and jumping threads is the OEM flash hider. no way to build it up and run a die down it because it’s shit diecast alloy.
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Yup, pretty standard Chinasize workmanship right there!
Thread tape is about the best way to be able to make the thread useable and still be able to attach/detach anything onto it.
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Can’t remove it from FSB at all?
From pics looks like it’s pinned to FSB, hard to tell.
Nope… not removable, one piece cast alloy.
The plate at the rear of the gas block that the upper and lower handguards key into is nylon and separates, the rest is just one big piece of potmetal.
Ah well… China 
I’ll probably end up lopping off that thread and turning up a sleeve that press fits onto the existing barrel and pinning a bit of decent OB with a CCW thread into it. There’s always a way.