I just measured mine.
The B&T rail is about 0.55" from the top surface down to the top of the receiver.
It's about 0.025" from the top of the rail down to the flat surface just forward of the windage screw.
I free handed these with my calipers, so there's a bit of error there, but it should give you an idea of where the rail sits.
As far as co-witnessing, I'm going to stick with my lower 1/4 cowitness description.
The open position is good to get a clear view over the sights and through the dot, and the night sights are visible, but when I try to use the notch, I tend to lose the rear sight against the T1 body.
It may be easier with the green finish on your 551, or maybe if the T1 was painted (or something to give you contrast).
The peep positions are usable, but you lose some of the peep's field of view to the curvature of the bottom edge of the T1 window, and that slanted portion in the lower right of the T1 is visible through the peep.
YMMV, of course.