In regards to the "F" and "I" stamped on the barrel lugs go here and pay particular attention to Richard Skeuse's response:
http://parkerguns.org/forums/showthread.php?t=5012
In regards to Parker frame sizes go to the PGCA Home page. In the left hand margin click on "Technical Information", a new page comes up, then click on "Frame Sizes by Gauge", the chart contains the info you're looking for.