I studied Japanese in Tokyo for a year, can speak and read the language, the character is definitely the Kanji for up, but turned upside down.
The kanji for down; 'Shita' is (下), which as you noted is the mirror image of the former.
I'm not 100% sure however that it had the exact significance I suggest, I'm just going with the most likely obvious meaning. It might also be a guide so that whoever is constructing these swords in a factory knows that the seppa goes 'above' the tsuba parts.
Of course, clarification by those with a better understanding would be appreciated, I'm still fairly new to gunto.
