No... Is it bantam eggs? They sometimes hatch a bit earlier than the usual 21 days because they're so small. Also if your incubator was running at a too high temperature the chicks may hatch a day or two early. But 6 days is very early.
My incubator is still air.I think it is about 30 years old I have not got any instructions with it .It stands on legs about 3 feet high is about 3 foot long and 2 foot wide.with a water Chanel running down the middle.It has air holes along both sides on the top.I didn't have any plugs for these when I got it.It seems to regulate the temperature by screwing a bellows system up and down I really have trouble getting the humidity over 43% .It needs to be 70% for hatching I think.I'm not sure how to do this
If you want to get humidity higher you can place some bowls with water in there. The bigger the water surface, the better it will work. Also wet sponges or a wet cloth. Your incubator sounds interesting!
My vintage redwood incubator would not get over about 40% either. I finally found some drawer organizers that fit perfectly and took up the entire space the original water pan did. With them all full it will go 50%. So I added about 36 cheap kitchen sponges, standing up on end in them and now it will go about 65%, spikes to 69% when there are wet chicks in there. Have been having a lot less shrink wrapped chicks since I have been able to do this. But you want the cheap cellulose sponges with big pores, foam sponges with tiny holes don't work, they don't evaporate the water like the cellulose ones do.