Long ago, Action Markers had a nice scenario style blowback semi called the AM-4 which used a collapsible AR buttstock attached via a clever captive threaded ring. I had always wanted to copy the mounting system for the Phantom, and finally got permission from Steve Deam Jr. himself to use it.
The valve is simply a CCI valve body with a captive threaded ring attached - the 6-position buttstock (shown fully collapsed) can be quickly removed without tools. The buffer tube is 'commercial spec' sized, so the stock itself can be swapped out if you prefer to use your own. Note that the stock and ring will interfere with the end cap on a standard stock class feed - if you want to run SC, you'll need a Stripes feedtube or other raised SC feed.