This 4 piece type plug was common in the 1920's thru early 1950's - its the way plugs were made before the modern crimp together or molded in insulator plugs came out.

The flush end with the side wire ground gap was common for racing applications. They came in a number of thread reaches and both 14 and 18 mm sizes and several heat ranges. This kind of plug would have been used in 4-60's etc. Motors as late as the early Konigs used 18 mm thread plugs with the side wire ground and the first plugs recommended for 20H's and 55H's were this type too.