These are some PF940C locking blocks that were manufactured using 4130 Steel when we were examining some different materials as a cost saving measure. 
 
They were then heat treated to a 38-42 Rockwell C to be clsoer in spec to the 17-4 rails. However we also experienced some warping from the Heat Treatment in certain cases.  
 
The 4130 in the HEAT TREATED condition are tough and abrasion resistant. They will last somewhere between the 4130 RAW Variant and the 17-4 Stainless Steel. We decided against using the material as in the end there were no real savings to be found and the heat treating resulted in warping in some cases. 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 make sense. These are the rails for the guys that like to tinker and massage and make the rails fit. 

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 OVERALL FOR THE FRAME DUE TO THE WARPAGE FROM HEAT TREAT. THE HEIGHT OF THE MAIN SLIDE CONNECTION RAILS MAY BE TOO TALL IN THE FRONT PREVENTING SLIDE INSTALLATION AND WILL NEED TO BE STONED DOWN. IF YOU ARE NOT COMFORTABLE WORKING ON THESE ASPECTS DO NOT BUY THEM! WE HAVE VERIFIED THEY CAN BE MADE TO FIT AND RUN SMOOTH AS BUTTER BUT THEY WILL TAKE A LITTLE EFFORT. THEY ARE $22.50 FOR A REASON!