The H&R Folding Model (large frame, heavy weight, 12, 16, 20 ga and .410 bore) was in production from 1928 to 1942 in two variations (1928-30 and 1931 - 42). The 'F' prefix to the serial number for this piece merely indicates "FOLDING MODEL" and that it was made prior to 1940. We don't have much info/data on number of pieces produced, so the serial number to year of manufacture would only be a guess and not very accurate. Pieces manufactured after 1940 will have the date code letter prefix followed by a large 'F', then the numerals.
There was also, a LIGHTWEIGHT Folding Model in 28 ga or .410 bore, made from 1925 to 1942, in two recognized variations (1925-30 and 1931-42.)