No internet for a day, and I finally get some likeminded feedback. To clarify, up close, the thing was definitely NOT a modified frame; it was clearly made for the wheels--the shift bosses, the chainstay/seatstay and forks were all built for 20" wheels.

Well biked, I didn't know that about the tubing; I didn't realize how nice that was. I was even tempted to go lift the thing up, just to gauge its weight, but didn't want to make an ass out of myself.

Unfortunately, I don't think I'm going to get to see this thing a third time. The first time, the thing was in motion and moving quickly through street traffic (I was on foot). The second time, I just got lucky and happened to be in the same area at the same time the owner happened there. If I see it again, I will definitely leave a note, but I think that's going to be a prayer.

Today, I actually walked by the same spot, and the bike was gone.

Arg. I'm going to PM T-Mar, and maybe send a query directly to Capt. Bike himself. Can't hurt to try, right?

And BTW, I believe, though I can't be sure, that the first time I saw the bike with rider on board, the rider was a young, hipster Japanese dude, maybe early 20s.