AC is an American sparkplug brand - AC Delco.
With the early AC plugs the number designates the thread size (3 = 10mm, 4 = 14mm and 5 = 18mm) and the letter (C) is the temperature range (how deep the gap between the central insulator and the outer metal casing is within the plug body on the sparking end).
I am not aware of AC doing imperial plugs but it is not beyond possibilities, details of these old plugs are hard to come by these days. A standard CM-6 will not have the same character as the lovely old AC plugs
so I would consider keeping them for when you show the engine.
If you buy a set of CM-6 plugs for it
make sure you thread them in by hand and only tighten the last fraction using a spanner.
Jo