I notice that Tamiya's M4A3 Sherman upper hull has a ridge along each side of the bottom edge. Pictures of actual tanks I've seen show such ridges with holes, suggesting a provision for side fender mounting. Is that the case, or is the plain ridge correctly depicted on the Tamiya hull? Thank you.

Your first assumption is correct. On the actual thing, there was a strip that was welded onto the side's lower corner edge. PE sheets have those parts included -- they would have you shave off that strip on the Tamiya kits.

Here's a partial pic of one on an early M4A3



From Toadman's tank pictures website