If there are eight "wheels", four of which have the letters A to Z and four of which have the digits 0 to 9, then there are 264*104 possible combinations, so the probability that a random guess will get the right combination is:
p=1(264×104)⇒p=0.0000000002188299
or about 2*10-10
Of course if a different alphabet is used that has a different number of letters, the probability will be different!