The symbol appears on p. 60 of John Stevens' classic Sacred Calligraphy of ther East (3rd ed.). The caption notes that it is a variation of the seed syllable VAMH, "Dainichi of the Diamond Realm.". In the book, the "A" has a long diacritical mark above it, and the "M" and the "H" both have dots underneath.

From the book: "Dainich Nyorai (Mahavairocana Tathagata). The supreme deity of esoteric Buddhism. Everything in the universe forms the body of the primal Buddha and he exists in the heart of all beings. Dainichi is the Great Sun illuminating all corners of space. All the other Buddhas spring from his immense energy...Dainichi has two spheres of action: The Diamond Realm, indestructibe, spiritual, unified, centered on prajna, and the Womb Realm (Garbadhatu), material, dynamic, fertile, all inclusive. The diamond aspect is shown here."