Chuck, thanks for the tip about the 44mm step-up ring working with Series-VI threads. That's good to know. I used to find myself wishing there were official step-up rings that went from Series-VI to other thread sizes (they don't), but if the 44mm works, then we're good to go!
Dave makes a good point too about adapting to the latest standards. But I think it depends on what kind of equipment you are already invested in. If you are working with a single camera which happens to have Series-VI threads (or no threads, so you are using a push-on adapter), then there's not necessarily a compelling reason to search for a Series mounted IR filter if you can easily adapt to a "standard" threaded size. This is especially true if you are already invested in a certain thread-on filter size for other cameras that you have.
On the other hand, if you’re like me and have a substantial number of vintage cameras with small non-threaded lenses, and in particular if you are already invested in Series filters, then it makes sense I think to go the Series filter route. Anyway, it sounds like you have a couple of choices to choose from now, so good luck with whatever you choose.
By the way, I find that a lot of older cameras look really cool with an IR filter attached - almost like a person wearing sunglasses. Here is my Duaflex III for example:
(Actually, Series-V is probably the more appropriate size for this camera, but as I said I am already invested in Series-VI so I used a V-to-VI step-up adapter).