So has anyone used one of these before?
I've seen a few photos of these online- It doesn't look like there's a stepped thread for a lockring.
I feel like this is not exactly for street use but moreso for chain alignment of a non/track hub or for racing on indoor track.

You are correct, sir. HED sells a similar one. It's intended solely to give you proper chainline on a non-redishable (read: disk) rear for track use (at least in the events that don't require back-pedaling). If it had a step-down thread for a lockring, you would just unthread the adaptor.

Do you know of any online resellers of them? My google search only brought up eBay listings...

Good point bout the lack of step down.- makes sence

the way he advertises them makes them really unsafe for someone wanting to do a street conversion....

^ and rightfully so

i used one on my first conversion. locktite on the hub/adaptor threads, then grease on the cog/adaptor, then a BB lockring. its basically a suicide hub for someone who's too lazy or doesn't want to bother redishing. i wouldn't recommend it, but it worked. fyi, i rode with a brake and learned to do skids on it with no problems. that wheelset is no longer in use, but i still can't get that cog off to save my life; with chainwhip or via the rotafixa method!