I am reproducibly getting the kind of rainbow-streak flare seen in the attached image on my copy of the Industar 50-2 (a russian 50mm f3.5 tessar-ish M42 pancake). The pattern does rotate when focusing the lens (which has a rotating front element).
Is this an effect inherent to the Industar design or manufacturing process, or a lucky manufacturing defect, or an acquired defect, and what is optically happening here?
There is no recognizable damage to the optics, nor are there notable performance problems with this copy.
Not looking for advice on fixing or avoiding the effect since it seems quite useful - if anything, looking for advice on how not to accidentally fix it.