Initials can mean different things, but these are initials appear to be siderographers and plate finishers. They typically appear in the margin adjacent to a corner stamp much like a plate number. From John Meek's article "Initials of Siderographers and Plate Finishers" in the February 1969 US Specialist, pages 66-68, there is a FAG = Frank A. Green and a CHR = Charles H. Roll both listed as plate finishers.
Add: Here are two more stamps with initials:
DWMcC = Daniel W. McCallum, siderographer (note, in LL corer)
RJL = Robert J Little
WWM - Willaim W Malone, plate finishers (note, in LR corner)