(1122:2:1) can give the set: x112,1x12,12x2,122x
There you have me totally confused. Take the second element from your set, 1x12. The two digits in their correct positions must be the first and last, no? So that means that the 1 in third position must exist in the target string, but at a different position than in the "guess" string.
Do you see the problem yet?