Lens is the Tammy 85/1.8 VC.
And you're right, put an APS-C lens on a 135 format sensor and you just get a gradual vignette that fades to black(ie. hard mechanical vignette). Some lenses can produce an image to the edge and just extremely dark.
But the inside ring pattern/size looks similar to what I used to get with my Sigma 10-20 when used on the D800, which I'd then crop to suit. Usually in a 1x1 format.