you mean that little pointy thing at the very corner?.. that's common for most backs.. I have the same thing in ALL my backs.. not to worry.. that's not a problem and has no fix.

In post #11 that pic has a light leak n that is either the mask screws, darkslide baffle, or foam needs replacing. Is teh foam gummy? take off the mask n check the darkslide bafflle to be sure it;s clean n not worn.. then when installing the mask again be sure to carefully snug the screws.

OH remember the image is reversed n upside down in your back (dull side to the lens) when trying to locate the leak. Orient the neg properly n the leak should be right there.