Is this a variant of the character ⿻丅口 (no mouth radical) shown in the evidence for UK-20685? Perhaps a mantra/dharani final syllable like "hūṃ"?

Yes, indeed, it is that character with a mouth radical. As you say, it may well be a mantra-final syllable similar to Buddhist hūṃ. I hope that we can find additional evidence explaining the usage and reading of ⿻丅口.