Great question man.
IMO, there is "2" Kush lines. One, as you stated, is the "original" Kush strains from the Paki/Afgan region. But, as we know, these particular mountain strains were grown for thier Hash making qualities, not so much for "bud".
Some of the Afgan/Paki strains were crossed, which gave us the Hindu Kush and the like.
Thats the "old skool" Kush.
Chem dirived "Kush" is actually just a "pheno" of the Original Chem '91.
Chemdawg is an actual name of a cat who used that as his "online name".
Basically, he bought a bag of herb from an a cat named "wonkanobe" and the bag had 13 seeds in it. After germination, "Chem 91" was "born".
So, back to your original question. Here is a quote from Chemdawg himself.
Chemdawg said:
'OG Kush' (the original cut) came from an s1 from in a bag of '91 Chemdawg in the Lake Tahoe area in 1996.
So, basically, the OG Kush lines are nothing but a freak pheno from a bag of Chem 91.
If you have ever smoked true Chem, you can get "hints" of that SUPER Kush flavor, with that mix of feuly lemoney goodness. Very Unique.
Now, that Original Cut of the OG has been crossed by many folks and there is now many different "OG Kushs" out there. But, when you smoke the real, you will know.
My 2 cents anyway, and that is the way I understand it. Believe, there has been many discussions about it, but that is the general concensus among Cali growers and breeders, one of them being DNA genetics and Swerve from Cali connect.