I am guessing they were for a British market, maybe "new castle" is the shot brand or its NEW castle as in improved
"
British shot sizes are usually given as a number although some of the larger shot used in wildfowling is "lettered" such as AAA and BB. The sizes and number of pellets per ounce used in the UK are given below, the smaller the number the larger the shot. The exceptions to this rule are for the lettered shot sizes which are reserved for the larger shot :- "
SHOT DIAMETER (INCHES) PELLETS PER OUNCE
SIZE
AAA ............0.20"................................. ..35
http://www.kwacs.org.uk/barrels&choke.htm