External H balancers and flexplates.
Having spun a few up over the years it is amazing and how sad it is on how far off most of them are from the factory spec. Especially the stock replacement type parts. The ones coming out of India are horrible. Perhaps even worse than China. The time zone difference must make a difference in the read outs.
The best replacements as far as weight is concerned have been the ones coming out of Australia..most of the time. Stuff does happen.
As far as race versions are concerned ATI hits it right almost every time. Fluid Dampners and ProRace (again Aussie) do well also. The rest are a big disappointment but they generally also exhibit bad machinework and have a history of falling apart anyway. Meow!
Funny thing is that the OEM replacements aren't much better when it comes to the weight. I have come to the conclusion at least as far as GM is concerned that we do not get the "first pick" parts as replacements over the counter.
Having an SFI sticker attached does not magically fix an incorrectly weighted part either.
I will refer to flexplates for the moment. The saddest things is that regardless of what country produces the item or whose name is attached it is unusual to find 3 flexplates in a row that match. I don't mean just a little different. A lot of times it's a lot different. Pretty sad.
The only cure is to internally balance everything but since that is not possible in every app I suggest spinning those parts up first to see if you are going to have some extreme grief finishing the job. You may want to correct the parts before continuing.
For what it's worth I don't blame a particular country for some of the junk we see. I blame the idiot that is located here and wrote the specifications and didn't pay for the quality control option.