Not sure if adaptor cord is available. But I figured this out-and my initial suspicion was correct.
Check out Audioquest's pricebook, page 42-43 on the link below:
You'll note that on the higher end power cables you can select a 20 amp cord. On page 43, it shows the different wall plugs as well as the jack that goes into your unit (niagara 7000 in my case). There is a different jack for 15 AMP and a different jack for the 20 AMP, but the wall plug is identical for either current.
I think the horizontal part on the outlet is made so people (or an electrician) can know from a distance that the outlet is a 20 amp circuit.
http://www.audioquest.com/wp-content...05_r15_Web.pdf
Agree with my theory?