This is the part of the thin strip that decorates the perimeter of the transformation scene.
It is decorated with a pattern of vajra (金剛杵) and lotus flowers. The vajra is a ritual implement that originated in India. It is said to have been a weapon used by the Indian Vedic deity Indra, and when Indra was incorporated into Buddhism throughout Asia as Śakrodevandra Śakra, the weapon became a ritual implement.
There are several types of vajras depending on the number of prongs extending from the pestle-shaped body. Thus, there is a single-pronged vajra, triple-pronged vajra, and quintuple-pronged vajra.