Bill

BA is a thread pattern, common here in the past, just like UNC UNF whitworth BSF etc.

2, 4 etc refers to the diamiter. in reverse order BA 0 being the largest down to BA 10.

http://www.chronos.ltd.uk/acatalog/Chronos_Catalogue_BA_Brass___Steel__Screws_295.html

Hope this helps

Best Regards