These are some PF940C locking blocks that were manufactured using 4130 Steel when we were examining some different materials as a cost saving measure.  
 
The 4130 in the heat raw condition is pretty tough and quite abrasion resistant and they will have a lifespan in below 17-4 Stainless Rails. In our opinion these are perfect for the 3-D Printing and Experimental guys as while you are getting many more rounds through a pistol these days they are still somewhat limited life span so the cheaper rails CAN make sense. These are the rails for the guys that have the patience to tinker and fit them and don't just want a drop in and go experience. They are good for training and gunsmithing practice. 

READ ME!!!  - NO RETURNS, WARRANTIES OR ADVICE GIVEN THESE RAILS ARE BLEM AND WILL VERY LIKELY NEED SOME WORK TO MAKE FUNCTION CORRECTLY AND FIT THE INNER CLEARANCE FOR THE BARREL LUG CONNECTION WILL NEED TO BE WIDENED AND SMOOTHED OUT AND THE RAILS MAY BE A LITTLE WIDE AND/OR WARPED OVERALL PREVENTING INITIAL FITMENT INTO THE FRAME IN SOME OF THEM THE RAILS MAY BE A LITTLE TALL PREVENTING SLIDE INSTALLATION. IF YOU ARE NOT COMFORTABLE WORKING ON THESE ASPECTS DO NOT BUY THEM! THEY ARE $20.00 FOR A REASON!