21. All patterns shall bear the number of the drawing on which they are
first detailed, followed by a serial letter, according to the number of
patterns on the drawing.
22. Standard patterns used repeatedly and liable to be ordered from in
repairs must not be changed. Other patterns may only be changed when
absolutely necessary and by order. When so changed they will bear the
original number and letter, followed by A for the first change, B for
the second change, and so on thus: 4860 AB is the second change in
pattern 4860 A.