I'm overhauling an early 1990s Bridgestone MB-1 that has been well loved, and customized, over the years. So the stem may not be original. In any case, I've never seen one like it and am hoping someone else has. in appearance it is a standard 1" quill stem, but instead of a regular allen-type quill bolt, inside the body (when looking down from top of bike) there's a soft black plastic cap that seems to be spring loaded. I flipped the bike over and it looked like i needed to loosen the quill from underneath--i could see there was a small nut on the end of a bolt (this is with the bike flipped over and looking down into the steer tube from between the forks). i loosened it what i judged to be enough, flipped the bike back over and gave the stem a gentle whack, but it didn't give. i wanted to seek sage council before i went any further.

has anyone seen a stem like this? advice on removal?