Again A + B

Consider the set of strings consisting only of lowercase Latin letters and decimal digits. Two such strings are called similar if:

1. They have the same length;

2. Characters at the same positions in both strings are either both letters or both digits.

For all strings similar to a given one, we define an addition operation. If all such strings are arranged in reverse lexicographical order, each string is assigned an ordinal number starting from zero. Let $N(A)$ be the ordinal number of string $A$, and $N(B)$ be the ordinal number of string $B$. Then, the sum of strings $A$ and $B$ is defined as the string whose ordinal number in this sequence is (N(A) + N(B)) \bmod M, where $M$ is the total number of strings similar to the ones being added.