i guess my old, dusty memory "remembered" correctly for a change. boogie
i dug out a 69 FSM and it showed the same choke mechanism as the one you have, including the side shot of it with the nut and the adjusting "lines".
however, when it came to telling how to adjust the choke, it did not show how to adjust the thermostatic spring, although it did mention that the spring was part of making the choke work correctly.
it mainly focused on slightly bending the choke rod and getting the vacuum pulloff set correctly.
it looks like adjusting the spring is a trial and error effort. make sure to mark the spring, nut, adjuster slot and body lines as they are now if you want to try adjusting the choke spring. that will insure you can get back to your original setting if need be.
if you decide to try this, keep us updated on how it works out, please.
beer