Ron the Coilman can possibly answer this best. I see brass top coils on ebay. Years ago I bought 4 coils at a farm auction and they have copper plates on top. I still have them somewhere but was wondering how common were the copper ones?
I might be able to answer that for you somewhat. The metal top coils came in 3 basic configurations not counting plating. There were the so-called 6 screw yellow brass top kind which were the first vintage followed by the 4 screw brass top and 4 screw RED BRASS versions. I have seen only a few more of the yellow brass type versus the red brass type among the 4 screw mounting types. By the 4 or 6 screw I am referring to the number of wood screws that hold the metal top itself to the wood box. I have NOT seen a red brass version with 6 screw mounting so to my mind the 4 screw red brass type were definitely later vintage and probably only slightly less common than the 4 screw yellow brass type. Many of the metal top types had anodizing or plating of some sorts but sometimes what appears to be a black anodizing is in fact just a ton of ordinary brass tarnish which can be polished right away. You need to look under the fiber 4 hole insulator sheet to see if the plating or anodizing is UNDER that fiber sheet to determine if the original top was just bare brass and just tarnished or whether it was perhaps originally plated. Ron Patterson and Trent Boggess wrote a complete article on coils in the Vintage Ford awhile back.
As usual John is right on the mark.
If you would like all the details about the history of the Model T ignition coil go to Vintage Ford Volume 34 Issue 4, 5 and 6.
Ron the Coilman
I would add that there are some differences between Ford and KW brass top coils. Ford switched to polished brass tops early in brass top coil production, but KW appears to have always used the black anodized finish, and KW brass tops appear to have always used six brass hold down screws. Many KW brass top coils have the letter T stamped into the back side of the coil about 1/2 inch tall.