I have an Maverick ET-7 which you can buy new today for $40 a little less than the ET-73. I think the difference is that the ET-7 is a two meat probe model and the ET-73 is a meat and smoker probe model. One of the ET-7 probes can be used as a smoker probe but once you have your GOSM door thermometer calibrated then the ET-7 may be a better for GOSM users. I find the GOSM temperature to be very stable once set so I really only check it when I am tending to the meat or the wood smoking box. I ran a test when breaking in my Big Block. I had one of my ET-7 probes in the middle of the bottom rack position and the other one in the middle of the top rack position. I monitored the two ET-7 probes as well as the door thermometer for 2-1/2 hours and they never varied over 5 degrees from each other. That is close enough for me to trust the door thermometer. Heck, if I want 210F then 200F to 220F probably be close enough. Afterall, what is a few degrees between friends? I recently cooked 3 back loin rib racks and 3 full loin rib racks. I placed one of each on a grate using 3 cooking grates. One of my ET-7 probes was in the top most full loin rib rack and the other probe was in the bottom most full loin rib rack. During the smoking process the bottom probe was always 3 to 4F degrees lower than the top probe.
To adjust the GOSM just follow what the GOSM manual says to do.
"Remove the heat indicator from the door of the smoker by loosening and removing the mounting nut from the inside. There is a second nut mounted on the back of the heat indicator that can be turned to adjust the heat indicator needle. Set the needle to the current outdoor ambient temperature and re-mount the heat indicator to the door of the smoker."
Now if you are an accuracy nut you may want to use boiling water and adjust your door thermometer to that. Please, lets not get into the discussion of what the boiling point of water is when, where, what, why, elevation, how pure, pressure, etc. The temperature of boiling water is where most of us like to smoke and the basic concept of a water smoker.