The RS-540SD Black Sprint motor was released three years after the 50136 RS-540SD Black motor.
The motors are identical except the "Sprint" name. The Black Endurance motor was also released in August 1983, with the "rebranded" Black Sprint, so I guess Tamiya had to put a more identifying name to the Black motor to make the difference obvious.
Only two kits were supplied with this motor:
• 58021 Can-Am Lola RM Mk.1
• 58032 Tornado RM Mk.3.