How Do You Date Spark Plugs

[gwhite]

Hey fellas, my apologies for the poor photo of the 1948-up plugs...here is a better one (this from a 1948 Canadian ad);



And a scan from a 1938 Champion catalog (shows the brass top design). Note: 14mm plugs such as the J8 shown below appear to have not used the brass top.



Here's the set of NOS Champion brass tops I picked up for my Winfield-powered "hot rod";