1} The numbers you see in a Osborn manual are the engineering numbers first assigned by engineers and designers. The part numbers come about when the parts division sources the parts from different suppliers and other manufacturers.

2} They may seem identical, but there is probably some slight, and maybe insignificant change to the part. Ford loves to save money, so they probably didn't change the part number without a reason.

1. Designed specifically for that particular year and model,

2. Designed for that model, but were carried-over 100% unchanged from the previous year,

3. Designed years ago for an entirely different platform, but used none-the-less on your Mustang because of it's easily universal application. Think about it, there was nothing "Mustang-unique" about many of the mechanicals on the car, like axle seals or lug nuts or C-4 transmissions for example (stuff commonly used across the entire Ford car line), so Ford surely wasn't going to bother re-numbering all of those general commodity parts just to make them "Mustang" parts.

An engineering # is assigned to a part the MOMENT that part is developed. The number will reflect the year and car platform that the particular piece is currently being designed for, but that same part may get picked up and used on MANY other Ford/Lincoln/Mercury platforms far into the future in order to save money time and resources. i.e., it would have been absolutely silly for a Ford engineer to develop brand new unique C8Z door hinge pins for the Mustang, when there were perfectly applicable and useable B9A door hinge pins developed in 1959 for the full-sized platform. So all Mustangs use B9A-5943030-AA door hinge pins that date back to "B" (the fifties) "9" (1959) and "A" (full-sized Ford passenger car platform). Many parts for the Mustang were "snatched" from the Ford parts bin. Thunderbird #'s ("S" body code), a BUNCH of Falcon #'s ("D" body code), and a bunch of 50's era passenger car ("A"). When it comes to wheel studs, and axles, and electrical items, the origin of the parts pre-date the origin of the Mustang by years and years. All Mustangs (and every other Ford car) were using 1949 Ford wheel studs in the 60's for instance. Only those items that were either developed and produced from scratch for the Mustang, or even needed just the slightest design change for the previous year Mustang received a new Mustang-specific #. Ford did not go to the gargantuan trouble of re-numbering every single part # every year for parts that were simply exact carry-overs from previous years, or came from a different platform.

The actual "Ford Part #" is assigned to represent the part in its final packaging, ready to sell. This is different than an engineering #. The engineering # identifies the part itself, before it has been installed on a car on the assembly line or stuck with a sticker or put in a box to be sold as a replacement part. The engineering # can be a "casting #" when it is actually molded into the tooling, or it can simply be a sticker when it is something that does not lend itself to having a permanent # stamped onto it (exterior windshield moldings for instance). The moment the part is put in a box or package, or simply LABELED for final identification, it recieves a unique "Ford part #" that tracks it in that state. A Ford part # can represent either a single part, or an entire assembly of numerous parts, like a transmission. The casting # on a tranny is ACTUALLY nothing more than the engineering # for the case itself.