in case anyone else wants one:
http://emrr.org.uk/cmmg-bhoa-bolt-hold-open-actuator/
Only works properly with cmmg's own mags, as the mag follower is a little different, the edge closest the trigger has a little cut out so the follower can lift the bhoa into the path of the bolt when the mag is empty - whereas other mags like black dog - the bolt hits the follower at it's highest point.
The difference being, with a cmmg mag -drop the mag out and the bolt stays in place, with black dogs the bolt goes into battery as soon as you drop the mag out (unless you hold it open with bolt release / bad lever).
I had cmmg gen3 mags on my mk4 cmmg (20" barrel fwiw) but they were utterly unreliable so I switched to black dogs, tbh I don't miss the bhoa that much though I do think occasionaly about modding the mags to make them compatible.....