It is not a proof mark. It is a maker's trade mark. It is the mark of George Laloux. Laloux was in business from 1881 to at least 1933. In 1910 his trade mark changed to a bee with GL in the wings.

Drop me a line at info@damascus-barrels.com. I wrote a short article for the Lefever Collectors a while back about the trade marks found on Lefevers. I will email you a pdf of it.

Pete