The letter F is the most common in the post war models made between 1950 - 1958, the seriel numbers are from 118 - 2325 with a smooth barrel and adjustable rear sight.
I believe the grips on your gun to be custom made and not the original, as the junior was only supplied with the "Chequered" grips.
The trouble with these letters are that they were used over and over again, they are batch letters, so a bit of detective work is needed to trace the year of manufacture. For instance "F" was used between 1946- 1950, but the rear sight was fixed not adjustable, same with the ones made pre war, but they had black tin handles.
I willl stop before I bore you all to death, a good book to try and get hold of is The collectors guide to Airpistols, by Dennis E Hiller.
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..If it don't move.....shoot it in case it tries to!!! 
