If it's an underwood - it should say underwood (or any other manufacturer) next to the rear sight. If it only says underwood on the barrel that doesn't really tell you anything about the reciever (the part by which manufacturer is defined). Under the Underwood should be something like 8-43 or so, giving the date of manufacture for the barrel. The bomb and P are US acceptance/proof marks. The arlington arms is most likely an import mark, if the gun was officially given to a friendly nation after WWII and later brought back in other than via CMP you find those.
F 135 - the right choice