@deepceleron: thanks for the info, i never tought about the problem like that.

Since it seems to be an interesting problem, i'll try to write a perl script to take a private key in WIF format with x missing letters as input, and permutate all allowed characters in this string, test if the permutated pk is valid and write all the valid keys to a logfile (or import them in bitcoin core directly).

Since this is a bit more difficult, and i can only work on the problem during my luchbreak, it might take a couple of days tough.