I was able to get pretty much the same things in the SCB as I planned to put in the MCB:
(with pink cap)
Contents: Pink folding baseball cap, Rick Steves guidebook, 18 oz. Klean Kanteen, long-sleeved shirt in a size small, a medium Double Org Pouch with my EDC sundries in it.
(Pink cap removed to show other contents
Note, there is not really much extra room, except for flat things like a folded piece of paper or map. I don't have any knitting WIPs in this test pack, for example, and it would be hard to stuff a rain jacket or extra snacks in there. (There's a bit of leeway at the top - I could probably pull out the shirt, slide a med OP with my knitting in it, and scrunch the shirt on top of all the rest of the stuff, with the flap to hold it closed. (but I haven't put a wallet in there yet)
So, I will have to be more deliberate about packing for a day of sightseeing - e.g., if we will be going into churches, I might wear a shawl as a coverup and tie it around my shoulders or waist the rest of the time. Or I can leave the guidebook at the hotel, or have DH/one of the kids put it in their pack.
Another thing the test pack made me realize is that beyond a certain load weight, I prefer to carry backpack-style (or possibly tote-style, if it has handles like the Shop Bags) - I don't like a heavy messenger bag. I also thought about using the PCBP most of the day, and having the SCB inside as a packing cube. Then, if we go into a place which doesn't allow backpacks, I can put most of my gear into the SCB, and wear the rest (jacket/shawl/things in pockets).
(I also returned the Ultrasuede strap - I didn't find it necessary for what I was carrying in the SCB, and I also found that it wasn't always in the position I needed - I switch sides a lot, and since the U/SS doesn't slide, I was having to snap and unsnap it a lot, or put up with a lump against my back. I might want it eventually for some other type of bag, but for now I don't need it. I also found that the waist strap, at least on the MCB, hit me at about mid-thigh. But I'm keeping it for now, thinking it may be useful for crowded situations where I don't want to worry about pickpockets sliding my bag away from me. We'll see how it works out.)
In summary: it was interesting to see how my thoughts about packing for this purpose changed. I thought I'd want the flexibility of more space, but realized that the form factor made more of a difference than I thought. I like the compact size of the SCB, I can keep it on me without a second thought (sitting down, walking, with or without a second bag - for instance a backpack or Shop Bag), whereas the MCB was definitely a "Point A to Point B" bag - strictly for transport, not something I wanted on me all the time. Yay for the TB crew for making the exchange process so seamless, and thanks to all of the Bihnions here for helping me work through this decision!