I'd rather stick humorously with the cooling fins explanation, but it would be nice to get an answer. So far no cigar. Getting serious...
In reply to Rob's citation of the waymil site:
"Small hexagon steel block has 15 holes in different diameters along with 5 slots along one edge for forming wire rivets."
But this doesn't explain how the slots are to be used.
Maybe their phrasing is misleading, so let's rephrase their description:
"Small hexagon steel block for forming wire rivets has 15 holes in different diameters along with 5 slots along one edge."
I think that is really what they intended to say, just physically describe the object. There is no answer to the slots question.
I was taught that rivets are made by heating one end of a wire to form a slight ball, cutting to length, inserting the rivet in the pieces to be joined, hammering the other end, finishing each end. This process works for me just like that. So how might the slots enter into this?
In reply to Ted's post, the slots are not deep and they are cut into the block at a 45 degree angle, not down the whole side. On my block the top of the slots measure 5mm, the face of the slots measure 5mm so the hypotenuses are about 7mm. What are you going to bend in a triangular cutout of 5x5x7 millimeters? Whatever it is, that's all you can do with them. The slots have different gaps but they are all of the same dimensions otherwise.