The #178 Kadee® Magne-Matic® Coupler features a "Scale" Metal Whisker® Coupler (a smaller head coupler which is more to scale and is patterned after the type 'E" couplers), insulated "Scale" Draft Gear Box and Lid with rear and center screw holes for mounting.
2-56 screws are the most commonly used for center hole.
0-48 or 0-80 screws are used for rear hole.
Screw length will very depending on the mounting situation.
It is not recommended to use a metal coupler directly on a metal surface or in a metal draft gearbox to avoid possible electrical crossover and shorting that may cause damage.