British Association screw threads, or BA screw threads, are a largely obsolete set of small screw threads, the largest being 0BA at 6 mm diameter. They were used by Lucas in many of the instruments and also found in the MG early models in handles, windscreens and many of the chrome items.
UNC and UNF are symbols of Unified Screw Thread Series
The Unified Thread Series was agreed on by the United States, Great Britain and Canada in 1949 to obtain interchangeability. Recently, modifications have been made to the original 1949 specifications for ease of use.
The Unified Thread Series is classified into coarse threads, fine threads and extra fine threads much like Metric screw threads are.
UNC: Is the symbol for Unified coarse pitch threads.
UNF: Is the symbol for Unified fine pitch threads.