I placed a thermistor on top of LT1004, fired up air con and did some logging of a LM399A-based 10V reference.
(Extracting data from the screen photo is pretty painful
Could somebody share the orifigal communication software and a user manual? Anyone?)
Playing with numbers I created a pretty close approximation of the value based on temperature (see PPM graph). So it seems like my meter has about 90 PPM/C.
It certanly worth improving. Install MAX6325 instead of LT1004? But I don't have equipment to calibrate it later, so it's probably not going to happen.
Upd: reattached the photos.