I found these pics on gymnasticscoaching.com (for which I now get a malware warning, so visit on your own risk).
Has anyone here used those?


*edit:found a video
PZC2eQ3ovWI
In the blog post they said:
Both are intended to simulate (somewhat) a handstand on Rings.The first unit is home made. The round parallettes are commercially available. Coach Jim Rand bought them at the WOGA Men’s competition last year.