That's the normal way Gardner marked the underlugs for decades. If the frame is an SB1, and the 20ga was the original barrel, then the 44mag barrel was fitted since 2008, check the FAQs, you can determine if it's an SB1 shotgun or SB2 rifle frame. I think the first 44mag extractors were later than 2005, but I don't know for sure. Roll stamps on the barrel mean nothing, the P is common, FWIW. If the 44mag isn't drilled and tapped for sights, it's a 2009 or later barrel, if it slugs over .430" in the groove, it's a pre 2007 barrel.
Tim