(1.) ID - The X-15 program used a 3-digit ID code to record information.
The first digit represents the specific X-15 flown. 1 = X-15-1, 2 = X-15-2 or X-15-2A, 3 = X-15-3.
The second digit represents the number of the specific X-15. This is for free flights only. Captive flights used a "C" in the second position. If a free flight resulted in an aborted launch, the second digit was represented by an "A".
The third digit represents the total number of times that specific X-15 had been carried aloft by either NB-52. This number incremented for each captive-carry, abort, and actual release.
Only X-15 free flights are shown on the above list.