To further clarify, this is what the roll bar phaser units looked like: (Cut and paste the link, perhaps with ? added to the end, to defeat hotlink-ban measures)
http://www.ex-astris-scientia.org/scans/mechanics/brattain1.jpg
At the lower right corner, you can see that the roll bar corner cylinders each had two "standard" single phaser turrets, surrounded by yellow markers just like the twin turrets on the hull were. The beams emanated from these turrets. There were no beams coming from the front ends of the cylinders, or from the aft ends.
Apparently, the front and aft ends weren't weapons - for all we know, they may have been navigational deflectors, considering they are shaped vaguely like the forward end of the
Enterprise secondary hull. At one point, a white light was also seen shining from the forward end.
Too bad that the TrekCore screencaps are very poor in this respect: they fail to show where the beams are really coming from. But by slo-mo checking of the actual movie, you can see how the beams come from the sides of the cylinders.
So, in total:
* three times two phasers on saucer top
* three times two phasers on saucer bottom
* two times one phaser on each rollbar corner
* just possibly two times one phaser below the impulse engine - these are only visible in a backstage photo of the model ventral side, and are removed (if they ever really were there) from all subsequent appearances of this model, as well as from all future
Miranda models
So, sixteen (or just possibly eighteen) phasers, plus four torpedo tubes, built into the model. On par with the decidedly eighteen phasers and two tubes of Kirk's ship...
Timo Saloniemi