A colleague working on Yunnan Sanskrit inscription showed me an ink rubbing recently (15th century?) and I noted the impossible character āṃḥ (middle of the three bīja letters in the attached image). This character is well known in the Japanese esoteric tradition and there are some scanty references to it in works by
Ōna and recently Rambelli and
Dolce. When I asked Prof. Kojima Giryū who specialises in Siddham characters some years ago, I was given the impression that this might have been a Japanese innovation. So it is interesting to find this in the Yunnan corpus as well. I am curious if anyone knows of any discussion about this impossible character in Sanskrit literature, especially in non-Buddhist Indian sources.