"Square" in this case means "rectangular", as you correctly point out, but without the "Gloucester" raised text, as other builders used very similar designs (externally, at least!). Ditto the "rounded" ones - they're intended to be able to represent the round-bottomed-Attock axleboxes, but also other round-bottomed grease axleboxes (such as the early GWC ones mentioned by @Compound2632). One of those cases where to represent all the possible varieties would have been impossible, but at least the shape will be correct, and those that wish to detail further (for example by adding the "GLOUCESTER" text, or a little dry-brushing of the edges of the round-bottomed 'boxes to represent the ridges on the Attock Patent box) can do so. The Ellis axleboxes, with their distinctive shape, weren't quite either, so got their own representation as they were quite common.