I know it won't do much good but when I grew 10 years ago I had received clones of pure G13, not the crosses that were floating around, but the REAL DEAL HOLYFIELD from the government lab. That was a very special plant, Northern lights was the other commercial strain I grew. As a comparison in the same soil and 1000 W hps the main stalk was 3 times the size of Northern Lights. I know that certain reports put the pure G13 up in the 30% range for THC and would agree with that.
I's a shame that you can't get seeds for it, clones only since 1988 but if you ever have the opportunity to get clones you will be pleased. Only draw back but even on 3 ft tall plants, I was lucky to get 1/8 oz per plant yield. The strain was the most apical dominant I had ever seen. Even 3 ft plant had the top and then maybe 4 lower branches, no real forking to speak of, even with LST it would never really bush out. As a comparison the NL would make about 1 oz per plant in that same room.
But the taste and high was UNREAL, tasted like orange dimatap (kiddy cough syrup) , super orange citrus, and it was the closest I ever found to a 1 hitter quitter, Most people would be fried from 2-4 tokes on a joint. The buzz was great, kind of a head high but a nice body tingle also. Just one look at that plant and you could tell that the government did some serious breeding/genetic modification to it, making it so vigorous and potent.
Not quite sure what the potency of NL #5 is/was but I'd estimate in the 15-20 range, but the the G13 was easily 2-3 times stronger...
Just my food for thought, shame it's about impossible to find clones.