I'm not convinced about that, linderpat.
I bought a some VW parts that fit exactly for my Boxster. They were about 1/4 of the price Porsche wanted to charge from me. And they are 100% identical.
Concerning the hydraulic lifters. I'm pretty shure the INAs are OEM.
Hydraulic cam followers
There are not so much companies that produce OEM hydraulic lifters in the world.
And yes, there are parts you can't get after market, because of contracts with suppliers. Learned that when looking for OEM brake rotors. In this case the OEM supplier was
SHW AG - Home
But i found a small manufacturer that does them in OEM quality and sold me some directly. They look 110% the same and the quality is really like OEM. Could also take a look at production. So no "china relabeling". ,) And gues what - some guys at ebay sell these too.
So i don't believe that the poeple you've listet can get OEM parts from suppliers if there are contracts form Porsche with the the supplier. And you can believe me that Porsche is very meticulous and restrict concerning contracts.
Regards from germany
Markus