Today I did the hot air gun repair to the 4503 meter and I'm very pleased to report that it worked a treat, suspect joints that under a microscope appeared to be flawed on the component side of the board (2 layer board) where perfect on the solder side and after treating the top of those joints with rosin and gently warming with the hot air gun, the joints flowed nicely. However, there were 2 slight casualties, 2 polyester caps 22nF each used as HF filters had their outer cases slightly blistered, but still work OK. I'll get these replaced soon with new ones arrive.
The residual 9Meg that resistance ranges were seeing has now completely been cured and the meter calibrated and is in complete agreement my other meters, "winner winner chicken dinner" as Dave would say. The photos below show it in resistance range with open leads and also measuring 4 x 10M resistors soldered together quickly to test the meters capability.