A client recently complain that they were being bullied by a large machine manufacturer (let's call them company X) over my client's use of company X's part numbers. My client makes replacement parts for many of the machines built by company X. Company X alleged that my client's use of the OEM part numbers were a violation of company X's intellectual property rights. In particular, company X claimed that the part numbers were actually trade secrets (and or) trademarks belonging to company X.
The original parts have exacting dimensions and are built to precisely fit the machines they were built for; as a result, company X labels each of these parts with a unique part number to ensure that the correct parts are used in the correct machine. As you can imagine, a complex machine can use, for example, several different coil springs each of which looks similar but each of which is slightly different. When one of these springs fail or otherwise needs replacing, the customer can order a replacement spring by simply asking for the part by the unique part number and be confident that they will be shipped the correct part. Its a good system for identifying parts – so good in fact that virtually every manufacturer uses this method to identify parts in a product.
The client's replacement parts are also made to exacting and precise standards and are built to fully function in place of company X's original parts. The client takes great pains to identify its products as non-OEM replacements for company X's parts. The client's website, brochures, price lists and promotional materials make it clear that it is not affiliated with Company X in any way and that the replacement parts are not sourced from Company X. However, since it is nearly impossible to order a replacement part without using the OEM's part number, the client's price lists do include the OEM part numbers to identify the parts which the customer needs replacing.
Certain manufacturers take the position that part numbers are proprietary, either as trademarks or trade secrets. While it is theoretically possible for a part number to be a trade secret, and while it is theoretically possible for a part number to be a trademark, it is legally impossible for a part number to be both a trade secret and a trademark simultaneously. It goes without saying that a trade secret is, first and foremost, a secret. Only a few people can know what the secret is. It seems odd that anyone would even attempt to make a part number secret (how can customers use them to order a part if its a secret and they don't know what the part number is), but assuming the part number remains secret, it can be protected as a trade secret. By contrast, it is well settled law that a trademark, by its very nature, must identify the good will of the trademark owner. The trademark cannot be a secret. Quite the opposite in fact. If the trademark is not registered, then it must be so well known to the buying public that they identify the trademark with the products or services offered in association with the mark. We trademark lawyers often say that the mark should be famous to be protected as a common law mark. Obviously, if the part number is secret, it can't be well known, let alone famous. In the case of registered trademarks, those marks are actually published in the national trademarks register: they therefore can't be secret.
But as a practical reality, can part numbers be protectable as trademarks? Usually, the answer is no. Part numbers, by their very nature, are descriptive in nature, i.e. they describe the part by identify it. Hence part number 34-0456 could describe a particular part (a spring) which is different than spring #34-0457 or #34-0455. Furthermore, its surprisingly difficult to promote a part number as a trademark. Your not likely to see splashy advertising for replacement spring #34-0457. If there are hundreds of replacement parts, then it would be a monumental task to promote and advertise each of them as a trademark. When manufacturers do promote a number as a trademark, it is almost always a model number rather than a part number. For example, the luxury car manufacturer BMW often promotes models like the 320i or 328i. Each of these models have literally thousands of parts – and you'll never see BMW promoting any of those parts with anything other than a blanket message to purchase genuine BMW parts. So while it is theoretically possible to protect part numbers as trademarks, in practice it is rarely done.
There is one way to build some level of trademark protection into part numbers. This involves incorporating a trademark into the actual part number. Let's assume one of your company's trademark is RAPID. Your part numbers could incorporate the word RAPID as part of the part number (such as RAPID0545-37 for example). While it is likely that this part number in itself is not a trademark, you might be successful in convincing a judge that some using the same part number might be guilty of trademark dilution. While it won't be an air tight case, at least you won't get laughed out of court. Of course, the natural response to this approach is to simply remove the trademark from the OEM part number reference. This would eliminate the possibility of trademark dilution.
So can part numbers be protected? It's theoretically possible, but in most cases it it practically imposible. Part numbers, while valuable tools, do not fit easily into any category of intellectual property.