Re: Buescher True Tone soprano compensator key.
I think you are simply referring to the upper stack float key.
The lower stack has one also. Between G# cup and F cup.
As with the upper stack situation, any key depressed in the lower stack will close the float key and also close G# if G# is activated.
On horns with a front F, the front F key is between B and the upper stack float key.
Yes, on a soprano, millimeter measurements can and do make a note play in tune or out of tune.
A good tech will playtest a horn and make final adjustments with a tuner prior to delivering to the owner. The average ones dont even bother. The bad ones do a very bad leak check and tick off players.
I myself got into refurbishing vintage horns the same way you are, by researching and questioning how things work.
Just an FYI. The first step at setting up a saxophone correctly, is getting upper stack Bflat key to move correctly with the lower stack float key. All the rest of the setup branches out from this point.