The JB series was made from 1936 to 1948, in the St. Johns factory. This according to www.ismacs.net/singer_sewing_machine_company. Billy (lostn51) has posted wonderful cleaning and servicing instructions in the Vintage Machines section of this board. I followed his directions to service my 1925 model 127, and they worked great.
You are very lucky to have this machine. You should definitely use it.