Thanks for the plans Steven. Are they from the 'Shipbuilder'?
Bill, the idea of a water discharge for washing ash residue makes sense for the Ceramic where the outlet is above the ash discharge, but on Olympic the mystery discharge is below, and possibly offset to one side.
I wonder instead if it's an air inlet to prevent a vacuum forming in the water pipe loop. We see similar features on the funnel water pipes and WC discharges.
Disclaimer: I'm not a plumber so I may be way off-base with this.