What does the circuit actually do?

[kiyu-git]

Hi there,
thank you for great jobs.
I want to know what the circuit actually do.
I found this circuit is a kind of instrumentation amp with 1.5V dc offset, but I can't find a some implementations of it.

1. What is a function of 100k trimpot which is connected with 1M resistor and ground?
2. Why is a resistor which is connected PIN8 and PIN 9 of LM324N 100K not but 10K (I guess it should be the same resistance as a resistor which is connected with PIN10 and ground, if the circuit is a kind of instrumentation amp).