I did get this message back from the seller:
"Thanks for the info. The number is definitely correct Jim."
That is something to think over. If there is a missed number in a series or sequence that you think should exist but doesn't, like this 5516 as an example of a 'gap', then it just might be that this IS a formerly undocumented number and pattern. Mr. L, while incredibly detailed and accurate (I have his book, of course), and exhaustive in research, cannot be claimed to be 100% accurate on all of the details every time, simply because he might not have had access to some information. Taking absolutely nothing away from his work, of course, and the incredible knowledge, first-hand memories, work, and documentation by current and former members including most of the regulars here. It is a possibility that the car 'did' come with something heretofore undocumented for whatever reason, but whether or not the current material is the OEM material or a replacement we'll likely never know. I really think the Club should have a good quality picture of the data plate in any case. Even if the data plate is an error, its an interesting error.
EDIT: What this might have been was some variation such as Studebaker did with its occasional forays into "Spring Colors" that were not always very well documented (sometimes hard to get a paint chip, or definitive information about which models and what exact period it was available until an original car turns up and proves, along with the factory build sheet, the accepted information wrong) Perhaps it was contemplated and withdrawn, but made it out on a car or two since it was already made up.