Initials of surnames with Saint (St.)

When writing the initials of a surname such as "St. John" (I will use the first name of Tom for an example), would those initials be written as 'TSJ' or 'TS' or 'TJ' or are all of these equally common?

Another question: Is the full form of the surname always "Saint John", or has the "St." become some sort of prefix?

Traditionally they would be written 'T. St J.', but modern practice in the British English zone is to not use periods, thus 'T St J', or possibly (less formally) 'TStJ'.

Author:Frederick Edward Molyneux St. John

This author wrote articles for the 1911 Encyclopædia Britannica. Articles attributed to this author are designated in EB1911 by the initials "M. St J."

Example (1)

Regarding formal style, a note to a scientific paper corrects capitalisation ('ST') and the omission of spaces:

@article{6da73ab7d99e423c9ee8fa0a2f9c9843, title = "Preparation of core-shell polymer colloid particles by encapsulation", author = "RH Ottewill and AB Schofield and JA Waters and NSTJ Williams", note = "Other: 4th author - initials should read N St J (lower case t) Williams"

Example (2)

Further, the surname, if written in full, is St John or St. John and (this is definitely worth knowing) in the UK, it is often pronounced 'Sinjun' or 'Sinjin'. St John (name)