Some references from Beer's "Encyclopedia of Tibetan Symbols and Motifs":
A single-pointed vajra represents both the central chan-
nel as Mt Meru's central axis, and the union of all duality
symbols conceived of as non-dual. These include wisdom and
compassion, emptiness and bliss, absolute and relative truths.
The deity Kalachakra is described as wearing a long rosary of
single-pointed vajras, although iconographically Kala-
chakra's rosary is more often depicted as a long chain of
five-pronged vajras with a crossed vajra at its lower centre.
The sharp point of Yamantaka's harpoon- which is de-
scribed as a single-pointed vajra - symbolises penetrative