Jude, the series plugs are indicated by having what appears to be bent clothes hanger wire between some of them. If by chance you have these post that you do. With the series plugs if one of them has gone bad none of the others will work as they are like old fashioned christmas tree lights. My old haynes manual states before 1980 they are indeed the filament or series type unless they have been converted. A simple 12 volt test light should indicate the bad one when you go from ground to each wire in the string. When you get to the wire that does not light your test light you have just passed the bad plug. Make sure you start at point from where you got the spark as that is the hot end of the string with an open glow plug further along. Strong possibility from your post. Also no more grounding the system out to test as can damage the relay and even possibly destroy a plug in the process.