Until the EV system became popular in the mid-1950's, the standard shutter speed sequence ran: 1/x where x=1, 2, 5, 10, 25, 50, 100, 250, 500 with a shift in mechanics between 1/10 and 1/25 and between 1/250 and 1/500. The EV system required that each shutter speed be double the previous one; given that mechanical shutters are at best accurate to 1/3 stop, I suspect they changed the labels and not the mechanics. At least I have tested several Voightlanders and Kodaks from the late 1950's where the shutter speeds are labeled 1/15, 1/30, 1/60 ..., but test 1/10, 1/25, 1/50 ... as under the old system.