There are several cases where the part builder in datasheets is completely vague about the context of what they are referring to. The datasheet only says this for the one without “B” on page 1 under the “Package” section of the part builder: -48 = ECX-1048 (datasheet link). While the other datasheet says -48B = ECX-1048B. There is no context as to what that indicates, however, I did investigate the datasheets and noticed very minor differences. The specs are within tolerance to be nearly identical, but there are some differences here and there. The one detail that made me see a difference was the minor differences in dimensions. Most likely these are close enough to be cross-compatible depending on the application. Always check the datasheets and if something is vague, investigate the datasheet for differences and then make your best guess as to what that detail is referring to. Most likely, this means a different package design in this case.
There are multiple manufacturers that do similar practices where they make slightly different package designs for parts from the same series. In fact, there are some cases where there are no differences between parts that have different part numbers simply because the manufacturer wanted multiple options for the same specs. If the datasheet details match up with no explanation for a change in part number, this may be the reason why. Many unexplained letters are internal codes specific to the manufacturer that cannot be deciphered due to it being proprietary. The datasheet usually calls these “internal codes”.