Hi. I have the same set up. The black knob uppermost is to change spray pattern to a solid dot. Only a quarter turn is required.
When I purchased my unit it came with lots of spares,some I'm yet to identify!
I bought a couple of air caps and needles from ebay. Search for Q-Tech Silverpro and...
That was a good buy. Had my eye on it myself but already have a 3 stage hvlp. I bought a new gun of ebay. The top one on this link.
https://www.ebay.co.uk/p/878283527
I offered 125 and they accepted. Good gun but the added bonus is it comes with 3 different spray needles. These are about 45...