My S2A travel kit is pretty much like Dave Parker's, but I usually also carry a 40mm and have the 135 instead of the 150, and I usually carry 5 backs, usually with three for zone system reasons, one with color, and one with a higher speed film (or film to be developed for higher speed). If I know I might need a big lens, I might bring a 300 or 500 and leave out the 200. My preferred shoulder bag of late for the S2A kit has been a large Perrin Kontur case made in the 1970s, which is sturdy, simple in design, and doesn't look like a modern photo bag.
Then when I'm out and about, I'm usually carrying only 1-3 lenses, depending on my shooting plans for that outing. If I'm using one lens, it's usually the 50mm. If I'm carrying three, it's usually 40/75/135 and a short extension tube for the 135. If I bring a flash, I might leave out the 75 and 135 and bring the 105mm leaf shutter lens instead. I often leave the bag at home and wear clothes with large pockets for my spare lenses and backs, but not a photo vest.