Here's a Wilcox shroud shown on a Team Wendy.
My old helmet and most Airsoft helmet have these shrouds (note it doesn't have the tab in the middle).
The Rhino and Rhino II mounts use the clip in J-Arms
Here's a Wilcox mount, showing the underside where the J-Arm/Device will connect.
The Wilcox mounts use a shoe to connect the the J-Arm vs the clip in. More secure I assume, but they're alot more expensive. The IR Patrols use the Wilcox shoe for example
There is a Wilcox Shoe to Clip-In adapter being sold, but it will drop the overall assembly .5-.75" I'd guess, so depending on where your monocular sits it may be fine.
I had initially read the Rhino II mount would not clip into the Wilcox mount, meaning I would have to get new J-Arms, but that was apparently incorrect.
Since this is bit of a tutorial on Monocular mounts, there's also a guy on ebay selling a clip-on J-Arm Mount for use with Pulsar 19/38/50 Monoculars.