I wouldn't be surprised that he got the cars traded when it was still possible. A reddit comment calculated it based on a 1:15 chance to get an octane in the first place, 1:12 for the paint, 1:4 for a certification and 1:15 for striker. 15x12x4x15=10800 I believe the current Import Drop has more than 15 import items, if you can figure out that amount, you can replace it with the first 15 and you should get the chance calculated more accurately.