Nothing more than a rear cap for M42 ols lenses.
But this one can be modified easily to fit the screw thread to the accuracy of your printer !
Here it is with an HELIOS 44-2 lens.
You may adapt the SCAD script just changing a bit this line :
clearenceM42 = 1.0; //add some play between nut and bolt
(If the screw is too tight increase the value (eg: clearenceM42 = 1.5; //add some play between nut and bolt)