In my original suggestion of this particular area... I was going on my experience with restoring a 1946 Willys Jeep... On it, the brake master cylinder was a brick shaped metal box with rounded corners and a pipe-fitting-threaded-cap that screwed INTO the top of the thing. It had a large square wrench fitting (picture a pvc pipe screw in endcap) on the reservior.
It was mounted just under the edge of the firewall (actually UNDER the body tub) - to the inside of the frame (inside the c-channel).
I had the good fortune to do all the brake line replacement work, and master cylinder rebuild (and engine and transmission) with the body tub not on the frame... Things are a lot more visible in those circumstances...
But, if I was having a hard time seeing it from under a complete car... I'd have someone press the brake pedal several times while I was looking at, and following the mechanical movements until I found it.