I presume by "frog" you are talking about the wedge-shaped moveable part that sits on a sloped ramp? The angle of the wedge has to be added to the angle of the bed ramp (wrt the sole) to arrive at the pitch angle. I have never measured the bed-ramp angle on any 'bedrocks' but eyeball memory suggests it's around 15 deg to the plane of the sole, so that plus the 30 gives you standard pitch.
If you have already allowed for the bed ramp angle, meaning the angle of the frog piece is actually about 15 degrees or so (hard to see how you'd fit the counter-sinks for the screws in such a thin bit of metal), then I have no idea what it was intended for. Thirty deg. is a "crossover" pitch - you could use a blade BD at that angle (just!) as long as the blade bevel angle doesn't exceed 25 degrees. This gives a miserly 5 deg. clearance, which is not really enough on many woods, 10 is usually the recommended minimum, which is one reason why very few BD planes are pitched at less than 35 deg.
Using a blade BU on a 30 deg bed wouldn't be much fun. That would give you a cutting angle of between 55 & 60 deg depending on your blade bevel angle and a plane that doesn't much like being pushed around......
Cheers,
Ian
