Cliff's book says this:
"Very early units have brass cup plugs. The leakage rate for these units is almost 100%..........Later units made after about 1968 used pressed-in aluminum plugs. We still see a few of these that leak, but not nearly as often as the early-style brass plugs. The method to install and swage in the aluminum plugs continued to improve through the years of production. It is quite rare to find units made from about 1975 and later that leak".
I hope this helps!
