Jim O'C : Congratulations, that 1680 F you pictured is a rare one for sure. I looked in my reference for uncatalogued sets but it only lists Firestone sets from 1954 nothing pre-war. I then checked the TCA publication "Lionel Trains Standard of the World 1900-1945" and Vol 1 (1976) had no listing of this configuration at all and Vol 2 (1989) still did not show it in the normal listings but in the Errata Section did in fact list your tanker and described it as shown by you (book has no picture) . The Greenberg pre-war book from 1983 in a very lengthy discussion of the Lionel lithographed cars seems to describe the configuration of your car but it is only 2-3 sentences and no picture, perhaps this book has been updated but I do not have a later copy. However, you may have an even more rare configuration as the TCA book states that tankers made in 1941-42 did NOT have handrails and yours clearly does. So if that is correct your car would date from 1940 which is consistent with your dating. WHAT A FIND!! I have been searching tankers for about 40 years and have never even seen one like your picture much less been able to acquire one. In truth before your picture I was unaware it even existed. Thank you for posting!
Don McErlean