I'm stumped. Walbro WA59 Carb rebuilding?

Hi guys,
 
I have a Walbro 59 (stamped in the carburetor), I'm putting it back together from pictures. I have a picture, not the greatest, despite being large, but it shows what appears to be some sort of 'plug' or 'cover' over one of the ports. I can't recall removing it, nor can I find it among all the parts in the container. So I'm not sure if it's gone walkabout or is something else altogether (screen maybe).
 
Originally I purchased this gasket kit but found it was not 100% correct. Iirc it had a couple of gaskets/diaphragms I needed but was also missing a couple. I then ordered this kit which seemed to be correct. Unfortunately neither kits have anything that will fit in that spot. The diameter appears to be 4mm.
 
Looking at an [https://www.jackssmalleng...9-1/wa-59-1-parts-list]online diagram[/url] (not sure what the '-1' means after WA59), it shows a possible something (#23 in the diagram) but there's no link to it, so that something is unknown. :/
 
 
Anyone happen to have this carburetor or know what I'm looking at?
 
Also, I noticed the metal 'plate assembly' (see attached pic) has a hole in the top-left corner, but the diaphragm and gasket doesn't. Why would this be? Wouldn't it just be blocked by the gasket? or likely the same part is used in a different model carb where it serves a function?
 
Appreciate any feedback on this dilemma :)