OH-stations using suffix /S or /SA indicate stations using old military equipment during special commemorative events, usually 4th June or 6th December each year, but these suffices can be on air other times as well.

"/S" stands for the Finnish word "sotilaskalusto" which means "military equipment".

"/SA"  stands for "sotilaskalusto aktiivikaytossa" which means "military equipment actually used in war".

Current rules state that equipment must be at least 40 years old to be used with /S and have actually been 

used in Finnish military operations during WW2 to be used with /SA.

These suffices may also be used with 
OG,  OI and OJ callsigns.

Thanks to OH6NT for the above information.

Club Log maps callsigns with the /S suffix by means of a software rule  and maps callsigns with the /SA suffix by means of callsign exceptions..