History
Parts books and trade catalogues
The trade catalogue is a reference work that happens to sell things. Its numbering systems and plates were built for people working with the goods in front of them.
Consumer catalogues are read by people deciding whether to buy. Trade and parts catalogues are read by people who have already bought, are holding the object, and need to identify one component of it correctly on the first attempt. That difference produces a distinct and highly disciplined kind of book.
The number is the product
In a parts book, the part number is not a reference to the item; for practical purposes it is the item. Descriptions exist to confirm that the right number has been found. Good numbering schemes carry structure, so that a family of related parts shares a stem and varies in a suffix, and so that a transposition produces a number that does not exist rather than one that exists and is wrong. That second property is worth a great deal and is often designed in deliberately.
Plate and list
The characteristic spread is a drawing on one side and a numbered list on the other. Separating them keeps the drawing clean, allows the list to carry columns the drawing has no room for, and lets the same plate serve several variants by changing only the list. It also means the drawing survives translation and revision without being redrawn, which is why plates in long-lived catalogues are often much older than the pages around them.
Indexes carry the burden
Because the reader may know the part's name, its function, its number, or only the assembly it came from, a serious parts catalogue carries several routes in: a numerical index, an alphabetical index of names, and a table of assemblies. The quality of a trade catalogue is better judged by its indexes than by its pages, and a book with only one route in will be slow for most of the people using it.
Revision as a design constraint
Trade catalogues change continuously, because ranges change and parts are superseded. That is why so many were built for replacement in sections, with dated page footers, section pagination that does not run through the whole book, and a supersession column pointing an obsolete number at its replacement. The catalogue is treated as a maintained document rather than a publication, and the design assumes it will be amended in place.
Why they read well now
Trade catalogues have aged better than most printed selling material because they were written to be checked. Their claims are dimensions, materials and compatibilities, all of which can be verified against the object. Anyone interested in how something was actually made will usually learn more from a parts list than from any amount of surrounding prose.