R: Residence qualification.
BP: Business premises qualification.
O: Occupation qualification.
HO: Qualification through husband's occupation.
NM: = Naval or Military voter.
RW: Residence qualification (women).
OW: Occupation qualification (women).
You might also see J or SJ which means qualified to sit on a court jury i.e. to perform jury service
Special juries were a particular type of jury which is now defunct.