Almost impossible to accurately date the older utility guns like this that were made without a serial number. These were cheap, no-frills firearms that were generally used hard rather than pampered and well cared for. I think you're going to be hard pressed to narrow down a manufacture date any closer than you already have with the above info. I would be inclined to say it was made in 1957 with the date stamp indicated, in which case the story of the guns heritage may be what's throwing a wrench in the works. We all know how the mind likes to play tricks on us as we age and the story has been passed down at least a couple generations at this point.