A yoga-teacher friend of mine just showed me a highly embellished picture of the yantra for anāhata cakra. I have attached the picture. As you can see, all of the 12 petals are represented with their bīja sounds, however, instead of खं, the second petal has व वं. I'm curious to know if this is a mistake on the artists behalf or is it an alternative bīja mantra, if so, why, and is there any textual source as to why this alternative symbol might be used. 

Thank you.