It seems that "sub registar office" is the term used in India and region to refer to what in the UK would be called registar's office or registry office.
If you put the term into google, you come up with loads of sites in India, Hindu net etc.
Sub would normally refer to a branch of a main office. It's a stab in the dark, but could this perhaps be a remnant of the days of the empire, when all offices in India would have been branches of the relevant offices in the UK? That's a guess, though.