Hello Moshe,
Harpa amouretta and H. amouretta crassa can be difficult to distinguish in a
photograph, because their general shape, color and markings are similer.
But when you hold them in your hand they are easy to separate.  H. amouretta
crassa is a very thick, heavy, solid form, while the typical H. amouretta is
rather thin and lightweight.  I think that H. amouretta crassa also has a
smaller length-to-width ratio, due to the increased thickness of the body
wall, and a smaller aperture relative to the shell size, for the same
reason.
Paul M.