I just posted a similar question on the maintenance thread, but came across this while scrolling through. I have the Flex 590 which comes with the collapsible, pistol grip tail stock. Had it at the range today, it is a sweet shotgun! I read the magazine reviews and various other posts about it, both pro and con, and I'm happy to say it is everything I wanted - including the ability to trick it out.
That said, my question on the other thread is similar to Tomcat's, although I found what my be the the answer for the Flex sling mount. I just bought a S.P.A.D.E (single point attachment device enhancement) from Slide Fire Solutions. You have to unscrew the locking nut from the slide tube, put the device on at that point, then screw the nut back down trapping the devise between the nut and the pistol grip. I'm just sure how tight the nut is, or if I need a special tool to loosen it.
Tomcat, I hope I didn't hijack your question, but I thought this could help answer your question.