R64B Imperf. Vertically?

I'm not naive enough to believe this is real, but I don't know enough to completely rule it out. I purchased this stamp several years ago when I was merely looking for an R64b. I didn't think there was anything unusual about it, and, based on the price, I'm sure the dealer didn't either. It was only much later when I realized that it should be imperforate horizontally, not vertically. It doesn't make sense to me that perfs would be added to an R64a (but who knows) and the side margins look too wide to be trimmed based on the R64c's I've seen. I'd be interested in any thoughts about what might have happened. Thanks.

Although it is early enough, it's trimmed, the perfs at top and bottom are genuine. Probably from a right margin example; the outside margins did not always get perforated. It might well have been trimmed to fit a document rather than for any fraudulent intention. A shame, it's a nice looking example.

Thanks very much for the response. I figured it had to be something like that. For future reference, how do you tell that a stamp has been trimmed? It's not obvious to me on this one.

Part of it is just knowing that this does not exist legitimately. Part of it is knowing ways and reasons that it might have been created, such as the possibility of a sheet margin.
In the end it comes down to 43 years of looking at revenues and having had the good fortune to interact regularly with a number of very knowledgeable revenue collectors during that time.

Thanks again. I guess I'm doing the interaction part now.