I got this Nikon enlarger lens that I noticed on delivery that it has a somewhat weird pattern on the rear element. Now I'm not lens defect expert, so I would ask for your expertise in analyzing this problem. I will post three pictures, one of the rear element at an angle, the second being a crop of the first showing the problem in detail and a third shot directly from behind the lens. The pattern consists of a series of disconnected problem areas covering about half of the rear element (outside surface). The problem areas look like cracks, very much like tiny pieces of dust or fabric that comes out of cotton. My initial reaction was for some sort of cracks caused by heat or impact, as there are no directional marks suggesting rubbing or abrasion. But I have never seen lens fungus, so I want your honest opinion on this. As a matter of fact, I've never seen anything like this on any of my lenses.