The supplementary fins are probably for additional stabilization as the vortex/coverage from the Shuttle will significantly influence the flow around the vertical stabilizer (guess based on memory of aerodynamics classes way back when)
from
http://www.astronautix.com/stages/nasa747.htm
Note speed and range
SCA Descriptions
Dimensions
Wingspan: 195 ft. 8 in.
Length: 231 ft. 10 in.
Height: Top of vertical stabilizer, 63 ft. 5 in.; To top of cockpit area, 32 ft. 1 in.
Weight: Basic weight, NASA 905, 318,053 lbs- NASA 911, 323,034 lbs; Maximum gross taxi weight, 713,000 lbs; maximum gross brake release weight, 710,000 lbs; maximum gross landing weight, 600,000 lbs
Engines
Four Pratt and Whitney JT9D-7J gas turbine engines, each producing 50,000 lbs of thrust
Performance
Airspeed limits with, and without an orbiter: 250 knots or Mach 0.6
Altitude: Typical cruise with orbiter, 13,000-15,000 ft MSL; typical cruise unmated, 24,000-26,000 ft MSL. Minimum temperature at altitude 15 degrees (F) (-9 degrees C) or 8 psi ambient pressure
Range : Typical mated, 1000 nautical miles (with reserves); maximum unmated, 5500 nautical miles
Fuel Capacity
47,210 gallons (316,307 lbs) jet fuel