I am sure there are a good bit of missingbacks out there.
I have never owned a 10 or 6 ohm Celestion. I have read about or noticed the oddball impedance speakers. I guess they have their place but need to be matched up somehow to a particular amplifier. I believe one was part of a Park combo or stack. I may have seen some on schematics or as part of the description in a price guide or maybe even Ebay.
They are interesting but I have never tracked any because of the odd impedance.
A 40Hz speaker is very interesting. But I wonder, is it full range or mid/bass?