The heater is an A.O. Smith model GUC 50 400, LO-NOX, naturally vented (not power vented); which doesn’t seem to show up when I google search it or search it on the water heater sites. Produced in Feb. 2016, installed in May 2016. It’s a 50gal. 400K BTU nat. gas heater in Calif. If a “dirt leg” is simply a branch at an elbow on the gas supply line designed to capture any debris flowing in the gas line; then, NO, there is not one. However, when I pulled the burner assy I removed the gas line from the valve to the burner and inspected the orifice for debris or spider web obstruction… NONE found. Again, the heater lights perfectly when cold, and only starts the cycling explosions when it reaches the preset temp. I apologize, I have an appointment to get to this morning, so, I don’t have time to get you the photo right now… I’ll post one when I return. Thanks for your interest and help.