While I expect Kube's answer is correct... We had a customer come in with the dovetail from a 1939 Mercury convertible, very similar (but slightly different) than the repro B- (which looked like it would work). The 48- definitely would NOT work in that car.
So there is a "gotcha" out there... whether all Mercury, some Mercury, or a limited production different part on a few Fords (aka the plastic jaws found on some 1937-38 Ford dovetails), or a convertible thing. I don't know. Just a heads up.