Bought A Fill Valve From China and Need Advice

I recently ordered a tank, fill valve and din fittings from China. I got the Acecare 2.17L tank which is very nice looking and the fill valve. The fill valve had some o-rings and other parts that I think I know how they assemble. But the way the pressure gauge turns as you open the valve seems strange to me and the red knob is loose. There are two set screws on either side of the red knob but either tighten or loosen the knob still seems loose. Hope I did not get burned on this valve purchase. The tank takes an M18x1.5 thread. Bill

 
I'm fairly certain that no O- ring goes inside the female DIN 300 connector. When you get your fill adapter you'll see a flat ring inside the male thread end that seals flush inside the 5/8-14 threaded DIN. If the green, black, and ivory colored O-rings are the same size, then they are just different material choices for the 18x1.5 neck seal of the tank valve. I've never gotten a red valve like yours but I suspect the tank pressure gauge isn't supposed to spin with the knob. You will get a quick response if you post a question to the seller you bought the valve from on Aliexpress. Go to your order number and you'll see a section where you can ask your seller a question. I correspond with sellers on Aliexpress and always get an answer within a day. Keep in mind the time difference as most time zones in China are 10-13 hours ahead of U.S. time zones.