Still the question is not clear.But from what I understood I can suggest :
If its a multiuser application , then you have to use a separate table (say ref_Reg ) reference to keep the last taken registration id ( say reg_RegId) . For the next registration increment ref_RegId in ref_Reg Table and select it, Concatenate the Id with / and the Year.
If the existing value in ref_Reg Table is 2 , then increment it to 3 and select into a string (say string refRegId )
string NewId =refRegId +" / " + DateTime.Now.Year;
in C#
Hoping this will help you :)