I was given a clone and told it was from Grand Daddy Purple Seeds from a trusted and reliable source.
Mine appears to be something of a mutant plant, and it was this one that failed to produce seeds.
I also did not quite get to 8 weeks before harvest (Grower's fault). May be plant is not good for seeding too. I am going to try again with a second clone shortly, and will run this plant until it drops. Maybe I will have better luck next run.
This is supposedly the source of the seed.