The box, which the previous owner provided with my B-50, is for a "L. R. Biggs iBeam Active Bridge Plate Transducer System for steel stringed acoustic guitar" It was purchased in September 2002 and presumably installed soon thereafter. My inference is that a plate is installed inside the body under the bridge using sticky tape and the endpin replaced with an endpin jack (that has a preamp). My B-50 also has the split saddle but since this is installed inside, under the bridge, I don't think that matters. (However it may be the reason this was selected instead of an under saddle pickup).
This is a link to my best guess for the "current" version.
https://www.lrbaggs.com/pickups/ibeam-acoustic-guitar-pickup
There is a recommendation to also use an external preamp or direct box and a comment about additional feedback control available by doing so.
When I use it, it sounds to me pretty much like the unamplified bass, only louder. I know that guitar players with good ears can listen to an amplified acoustic guitar and identify the pickup technology, but that is not me. I can tell you it works but I have no idea whether it works better than any alternatives or whether I could hear the difference.
As noted, feedback is a PITA and if I had to play the B-50 amplified a lot, I would invest in a preamp that hooked to my belt (or somewhere close by) and let me control volume and do some EQ without walking back to the amp. I'd probably need one anyway if I was going direct into a house system.
Like I said, the ergonomics are such that if I am amplified I'd much rather use an electric bass than the B-50 which, for me, requires changes in playing posture.
I have seen a couple guitar rosettes chewed up by removable soundhole pickups so, if you go that way, please get a "permanent" installation.