I was afraid that some sort of mask is needed. According to the pictures there doesn't seem to be any. What is that mask anyway? Piece of metal? Does it have any other purpose besides preventing light escaping around the negative?

Durst is still in mail, should be here during next week. I have 63mm and 80mm El-Nikkors waiting for it. Enlarging starts with 6x6cm and 6x4.5cm formats.