'54 Dominator rear sprocket

I have just stripped our the brakes on my bike, prior to going to the Begonia Rally. The linings were fine, but, for no particular reason, I counted the teeth on the rear sprocket. Having done this three times, to make sure! I find there are 42 teeth, when I expected 43. This must give a slightly higher gear ratio, and may account for me being able to exceed 70 m.p.h. in 3rd gear, but not rev out in top. Would this sprocket be an original fitting, and, is it worth the bother of changing it?