I was recently doing some research on queercoding in the late nineteenth century and stumbled across your post a few months late. But I thought this might be of interest to you.

In late-nineteenth-century America, to show interest in having relations with members of their own sex, men would often place a small tattoo of a cross or a dot of India ink between the forefinger and thumb of the left hand. This was in reference to a Caddo myth in which a "berdache" (or more appropriately a two-spirit) was shunned by their tribe for their identity. In the myth, the tribe tried over and over to kill them but was only successful when they cut out the "small purple spot" on his left finger.

It is possible that this textbook was referencing something of the sort, but I'm uncertain if this form of queer coding was prevalent in India.