It is a German/European means of signifying a polygonally-rifled barrel. My HK45 has it on the side of the barrel exposed through the ejection port when in battery also. Most, if not all, of my (Austrian) Glocks have it also. Over the last few years, Glock has dropped a lot of the European features (markings) for US-destined pistols. Austrian proofs went a long time ago, more recently the "US" suffix at the end of the serial number went, etc.
Linear Mode
