Mueller Insulated Plug-On Test Prod with Stainless Steel Tip Insulated plug-on test prods with 2 mm stainless steel tip and nylon housing. (Stock numbers 714-8207 and 714-8216 have a 8-32 thread on the point).These test prods are compatible with both insulated, non insulated or retractable 4mm banana plugs which are connected via a 4mm nickel plated brass banana jack. The length of these plug-on test prods is 83mm and they have a safety shock guard collar which is flat on one side to prevent rolling.