In the late 1950's, when radio licenses for fire and police operations were issued, many communities were assigned shared radio  channels.  Southborough, Marlborough  and Hudson Fire Departments all were assigned to 46.420  MHz.  To avoid the confusion of three Engine 1's, three Engine 2's, etc., the three communities got together and assigned banks of numbers: Marlborough was assigned 1-9, Hudson 10-19 and Southborough 20-29.

To this day, the tradition of Southborough using the 20 series continues.