The 1964 Johnson 5.5 has a fixed high speed jet (orifice plug) and an adjustable low speed. It sounds like someone changed the correct plug screw that had a post protruding from it to a plug with just the hex and possibly a screwdriver slot. Many fixed jet carbs used the slotted hex plug and to my knowledge the 64 Johnson is the only one that used the plug with the post. The post is for a plastic collar to rotate around and adjust the low speed via a linkage. See the picture below. #68 is the correct carb bowl plug screw for your motor.
-Ben
As I was typing, Mumbles posted the same reply. 😆
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