question: How many months after being flown in the testbed did the V-1710-C6 successfully completed the USAAC 150 hour Type certificate?
#1 return the  V-1710-C6 was  flown in  the   testbed 
#2 return the  V-1710-C6 successfully completed the   USAAC 150 hour Type certificate 
#3 return when  was  #1 
#4 return when  was  #2 
#5 return difference of #4 and  #3 in  months