I was clicking blindly one day and suddenly saw the Master Bus (along with the rest of that group) as tracks in the track view window. Now, for the life of me, I can't find the shortcut or menu command that toggles that on and off. Searched the forum, with no luck.
 
I want to be able to hand-massage the envelopes assigned to the Master Bus. (Producer X3, if it makes any difference.)

There is a little icon at the bottom of the track view that opens the busses, where the track status is (not the timeline).  Try that.  Once it is opened the buss heights operate just like track heights.
 
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You can also use the Shift+B shortcut.
 
 

Or drag the bottom divider up.  

I am sorry you had this trouble.  I don't know how you have been living without this.
 
I routinely put instrument groups in buses (e.g., vocals, harmonies, drums, bass, guitar, keyboards, orchestra, etc.) so I can create volume envelopes in the buses.  That way once the mix is set, if the overall volume needs to go up or down, or if one particular group needs to be louder or quieter, I can just change one node in each bus instead of fixing lots of noes in lots of tracks.  (I normally default Master to 0db but the groups to some value below 0db so I have some headroom.  I never touch the Master level unless there is a fade.