This is a special 1/4-28 X .571 dime slotted undercut flat head machine screw manufactured by a US based manufacturer we have worked closely with for more than two decades, made using 4037 steel and further processed with glass bead and rust inhibitor for optimal performance in our customer's application.  Parts like this typically carry a lead time of 6-10 weeks, depending on the secondary services involved and the quantity required.