FWIW The after market hammer spurs come in two widths for pre and post 1983 Marlins. Make sure get the correct one.
Further the Marlins that are even older, from the 50s and earlier, do not have hammers intended for the spurs. They are not grooved for them. If a spur were to be fit, it would wobble and most likely not stay on.
While the spurs are functional and helpful for thumbing the hammer beneath the rear scope bell, they often come loose and are lost in the field. The chance of this happening is directly proportional the the age, the value, the cost of the spur, and the difficulty of finding an identical replacement.