As I said these are undocumented. Basically this particular explorer decided to use their own convention for naming these types of scripts and never told anyone how they are doing it or what they mean.
If I had to guess, they are probably computing the hash of the script (possibly SHA1 or MD5) then enter that smaller hash in their database to search based on a hash. The starting letter (s, m, ...) may be a quick indicator on where in the database to look them up.