Serial # 178351 H under serial # 6sided star over it FN in oval above star. Any thought on age and date of manufacture?
I don’t know what the H stamped below the serial number indicates. But when H and L were used as part of the serial number, they were prefixes. L for light, H for heavy (I guess. L was the Light Twelve). But the numbers only ran to 100,000. So as Win7stw said, your gun was made in 1931. And it’s definitely not a Light Twelve as those debuted in 1946.