Your 28ga. 26" barrel set factory-choked M/F are scarce, but we have seen them on occasion. I recall there was a similar 2-bbl. set in this configuration listed on GI not all that long ago -- maybe this is that set?
Back in 2011 Richard Skeuse gave us an explanation of the F stamp on the barrel lug in this thread:
http://parkerguns.org/forums/showthread.php?t=5012