I believe these symbols are generally referred to simply as 'end marks' or 'end signs'.

At the publishing company I work for, we call them "bugs." I've also heard them referred to as "end quads," presumably when the character is a square.

Our creative team has always referred to the trademark and copyright attributions that we have to add to product names and various branding as 'bugs'. I guess it really depends on the industry you're in.

Are you referring, by any chance, to the section sign?

I guess we are referring to the # (end section) sign, as above.

george