Gurnard. The screw you're showing is the idle mixture which of course will affect idle speed when it's turned but I think Matt is looking for adjustment of the base idle speed. Your setup is the same as on my Mercury 15 Gurnard and the idle speed screw is on the left of the carb (as referenced in your pic) and just behind the fuel pump. It is a cam type thing with a brass headed screw with a spring round it.
"a" in this diagram...
But of course Matt's is missing that whole assy. There were some models where the idle was factory set from new and there was no variation possible... the manual refers to this possibility. The other thing is unlike ours Matt's is on remotes. Is it possible there is some adjustment of idle speed possible at the remote levers? Does it have a fast idle facility at the remote end.
I wonder if the throttle remote lever or cable assy isn't allowing the throttle at the carb end to fully return to the idle posn.