Please do not give up hope of learning the significance of the number "12".
It is too soon to pretend we know something yet, but it is also too soon to pretend that we will never know. We can even begin by pretending that we know something about normal practices. Mario Boano came from working at Ghia. From directing work at Ghia! He would have known about the numbering system employed by Ghia. He would have known more than we do currently about how that worked. He would have known that there was a useful purpose in having a numbering system. We can also presume that he would have been thinking "bigger" than doing production only with an eye to needing body numbers in two digits. If the number "12" reflects some sort of body body number, we can almost postulate that the number "12" is an abbreviation. We can be almost certain that it is not a "Series 1, number 2" body. It could certainly be a "number 12" body of a series of similar bodies. This would not surprise me at all. This would be a system very similar to that which was used by Ghia.
Now there are additional questions. Does the number "12" appear on parts of both seats and not just one of them? If so, we can eliminate the possibility that the seat maker numbered the seats individually, piece by piece, as he did the work. We cannot eliminate the possibility that the seat maker numbered his work in pairs. And we do not know that the seat-maker was working in-house at Boano. This could have been sub-contracted work?
It would be most "normal" if the number "12" reflects either a body number or a portion of it. It could be reflective of a true body number "12" or it could be the last two digits of a longer number. This is why it is important to look for clues in everything. From what we think we know of Boano production, it is not likely that the body is the 12th Boano body built, but it might be the 12th in a series or even the 12th body built after body numbering began. It could also be an arbitrary tracking number assigned by the folks doing the work but without an overall Boano format. This is not likely unless the seats were made by a subcontractor for Boano, as noted above.
If you wish to learn the significance of the "12", you will wish to make a serious effort to find it again somewhere in the car. You will wish to try to look at some additional Boano-bodied cars from the same time period of 1955-1956. Such a list could include a few Alfa Romeo 1900C, a couple of Chrysler Corp. specials, a few early Ferrari 250GT, some additional Fiat 1100/103, some Fiat 600, a Fiat 1400, a Jaguar XK140, a Lancia Appia and even the Lincoln "Indianapolis". There probably are others. Finding them will be one task and then getting permission to look them over (and under) carefully will be another. Even though they are earlier, there are a number of Abarth cars from 1955 that could be looked at carefully.
It is also possible that the coachbuilder "Ellena", in taking over the work of Boano, might have continued a numbering system begun by Boano? If that possibility should be found to be supportable in any way, there might be something to be learned by identifying the earliest Ellena body number ... if there is one. And, maybe you will wish to locate and speak to any workers who might still be able to recall such mundane details from 55 years ago.
Thanks for sharing a hint at a beginning of what should be a study. It remains to be seen how quickly another number can be added to the study, whether or not it seems to "make sense" when looked at together with the number "12" identified so far.
I have made none of the statements here as "proposed guesses". We don't know enough to make a real guess yet. But, we can consider some possibilities as being more likely than others while still keeping our minds open to all possibilities. When another number is identified, and if we can place an approximate date on that number, we will have another set of possibilities that may agree only with some of the many possibilities we have in front of us now. Add a third number and a fourth and ... maybe a pattern will emerge? Or not? Only then will we be able to make some real guesses ... that will still be guesses. After enough study, maybe we will begin to think we know something?