*EDIT: someone has pointed out to me that this looks like a font-rendering problem and is probably not a kanji. I’d like to clarify that this is not a font-rendering problem unless the box has two lines going through its diagonals. You can copy and paste it onto wiktionary.
EDIT: Oh, is that actually it? It doesn’t seem like the stroke thickness or anything is actually structured to look like a kanji, so I figured it was just a blank character.
I’m not really sure what you mean by ‘the abbreviation in question’, but it’s essentially just a pictogram of the box. I think the kokuji version is just a phono-semantic composition or something like that.
tbh, I was just going off the entry in the wiktionary page, but I can see how saying that the pictogram is an abbreviation of the kokuji might be miseading.
Yeah, I guess. Probably because it’s archaic no font designer bothered to make a Mincho version of the thing, but I see things like the digraphs for yori and koto and I wonder.
Given that hiragana and katakana originated as scribal abbreviations like the ones you’re mentioning, it would make sense to discount them along with the rice-box thing from being kanji, I suppose. I really only included it and the tangut thing for interest value.