Richard sent me this pic of a Japanese sword asking if the symbols are Siddhaṃ. They are, but there is a bit of a mystery. The top two symbols are hāṃ and maṃ which are both associated with Fudō Myōō.
The bottom syllable looks like Siddhaṃ as well but it is so stylised that I can't read it. Taking a guess I think it might be some variation on 'a' because that would fit what I know about Fudō and his association with Mahāvairocana of whom the 'a' is the bīja. My guess is that it is āṃḥ - the elaborate variation that includes a, ā, aṃ, and aḥ symbolising the four stages of the spiritual path in the Mahāvairocana Abhisaṃbodhi Tantra.
If you know for sure what it is please let me know and I'll pass it on. The original photo is here.