That is milled, not cast nor forged.
Not sure how well the milling was done as it appears the selector detent is misaligned (in both positions in the pics, the selector isn't straight) and it appears the hammer/trigger anti rotation pin needed a little bend to fit. None of this would prohibit running well...
The 308 conversion in the mg34 can be finicky, but when done right they just run. I've been doing 308/x51 conversions for a while with great results, long and short, and my personal short 34 (mg34s look) runs perfect at 950RPM.
Doesn't sound like you have a belt issue by your description, but the MG3 belts are greatly preferred for feeding 308/x51.
John U is an outstanding human being even though you say "I don't think he is a thief". You can't just say that he isn't a thief and his own guns are missing.... It should be obvious to anyone who reads this that there is a lot more to the story. I know a fair bit of the story and none involves John stealing his own guns.