I measured the voltage of all these DMM’s and they varied between 2.97V and 3.01V with nearly equal distribution.
My take on this is that it is not a high precision reference. (that’s why there is the pot). From what I have read about the chip. the Intersil ICL7106, it is probably not the reference voltage created inside the chip. I suspect that voltage to be actually lower, probably in the vicinity of 1.2V. Also that ICL7106 can optionally use an external voltage reference such as the ICL8069 mentioned by HKJ.
Also while researching, I found something that I thought was VERY interesting. The ICL7106, the very first “DMM on a chip” complete with drivers for LCD displays, was originally developed jointly by Intersil and FLUKE for a line of 3 1/2 digit DMM’s. It was the first meter to use a “DMM on a chip”. Intersil later made a few changes to the design and released it on their own.
FLUKE sued.
Later FLUKE developed a 4 1/2 digit chip on their own. That ICL7106 is now used by many, many manufacturers of cheap DMM’s and the chip itself has been cloned over and over.