Winch numbering relates to the power ratio of the winch. That is the highest gear ratio multiplied by the length of the winch handle over the radius of the drum plus half the rope diameter. That means that on a size 48 winch turning a winch handle through 48 inches of arc will pull the rope in by 1 inch. There is actually no direct relationship between the physical size of the winch and the size number, as in practice, some winch drums can be used for several different ‘sizes’ of winch.
The numbering system appears different because the ‘old ‘ style 2 speed winches were not true 2 speeds as the drive in one direction was un geared, that is 1:1. The only geared speed was therefore much lower ratio than new style winches which have 2 geared ratios. Improvements in the design of self tailing mechanisms and drum design have lead to smaller drums. Also improvements in rope technology has lead to smaller diameter ropes.
The result is that a modern 48 size winch is apparently half the size of the old 48 size. Always go by the numbering when replacing old winches with new unless you are looking for more power. Replacing with similar sized winches can lead to overpowering and damaging the rest of the deck gear.