It looks like a T-2 mount lens with an Exakta adapter, though it could be aligned forTopcon. If it is a T-2 mount lens, the rear knurled band is part of the adapter and the adapter will screw off leaving a 44mm male thread on the lens.
They are actually the same mount except that the Topcon variant rotates the mount about 90 degrees. Exakta had a release button on the left side of the body and when they added their semi-auto, and later full auto, diaphragm mechanism they had to use an external connection to the shutter release button. Topcon copies the mount, but put the shutter release on the more conventional right side which requires the "top" of the mount to be different.
With T-2 adapters, as opposed to the original T version, you can loosen the 3 grub screws around the adapters knurled sides and rotate the two parts of the adapter to properly align the scales on the lens to the top.