[DonRotolo]

I also think it's a bonded process.

What's messing you up is the rough side vs the smooth side, and this may be due to a completely different process issue, relating to how the fins are made (not how they are attached to the body).

How they are made: I am speculating that they are made by a sputtering type process, where the metal is melted and sprayed (as a liquid) onto a flat plate. The back is as smooth as the plate, the front is rough. This is very very fast when forming thin plates and gives a different strength profile compared to rolled-out plates of metal. The rough surface somewhat enhances the heat radiation since it has a greater surface area.

You need the small curve to cover the slot in the mold that the fins fit in to, so the base 'slab' can be poured over it all. If it was straight, it would either need to fit very closely or you'd have to allow some metal to dribble down into the fin cavity. Speculation again, though.

Yes, depth of field is the term. To increase the depth of field, you use a smaller aperture (on a camera, F16 versus F5.6). So, try putting a metal plate with a 1mm (more or less - experiment!) hole in it in front of (against) the lens. You will need more light for an equivalent brightness, but the tradeoff is a greater depth of field. 0.1 mm would not be too small, but lighting some objects may be tricky.