Welcome John and congratulations on your 1st post, hopefully it will lead to many more!
According to a post by Richard Skeuse, he informed us that an early batch of 28-ga. Repros had an F stamped just as your gun does. Others have reported that Repros were usually brought in in multiples or batches of 25 guns. So if that's correct, your gun may be 1 of 25 (or perhaps more) stamped with an F on the rear lug. We've discussed them previously here:
http://parkerguns.org/forums/showthread.php?t=5012r
How about doing a little show & tell about your Parker Reproduction in the "Let's see some wood" thread?