1. The outer left frame line is broken at the top just as it meets the frame line across the top.
2. There is a short line just above the lower outer frameline that makes the outer frame line appear a bit thicker.
3. The base of the bust should be laying on the inner frame line.

Detail for points 1 & 2 above

These are refered to as "marcas secretas" (secret marks) in the Tort guides.

Of course these cannot be 100% foolproof. Sperati's forgeries also included these marks. In this case you must check the stamp out by it's plating position. Sperati made his forgeries using a stamp from position #10.

Actually, there is not much to say about this forgery. There is absolutely nothing that compares to the genuine stamp. It is so bad, I doubt that anyone would mistake it for anything but an ugly forgery.