Help decipher DC brushed motor ratings
50 RPM suggests that the motor is actually a gear motor.  So the rated speed of the gearmotor is 50 RPM, the rated torque is 15 in-lbs and the rated current is 0.9 A.  As a sanity check, 50 RPM at 15 in-lbs gives you an output power of ~8.9 W.  The input power is 24 V * 0.9 A = 21.6 W.  That gives an efficiency of 41.2% at the rated load, which is not unreasonable for a gearmotor of that size.

Motor regulation is a figure of merit for DC motors.  It can be thought of in a number of different ways.  Some people like to think of it as the slope of the speed torque curve.  What it tells you is the motor's ability to turn electrical power into mechanical power.  It can be calculated by \$\frac{R}{k_t^2}\$, where \$R\$ is the terminal resistance of the motor and \$k_t\$ is the torque constant of the motor. A lower value indicates better heat conversion.  

Some people also use a similar figure of merit called the motor constant (\$K_m\$).  \$K_m = \frac{k_t}{\sqrt{R}}\$.  As you can see, this is just the reciprocal of the square root of the motor regulation that I talked about in the previous paragraph.