See the cracks right near the end of the cutout? The top one goes all the way through, and the bottom one is halfway across. In other words, this seatpost clamp isn't going to clamp a seatpost, no way, no how.
This is a Cannondale clamp, circa 1999-2000, although I can't say whether they manufactured these in-house or just bought them. A lot of their Coda-branded bits of that era were just bought and relabeled (for example, their Coda brakes were just Tektros), so my guess is that this wasn't Hecho en Bedford.
Regardless, this is a case where the cheap/light/strong triangle (where the designer is tasked to "pick two") has big "cheap" and "light" sides, but maybe not so much on the "strong" side. I saw plenty of these clamps fail back in the day, though the torque required to make them hold usually stripped the threads long before the clamp itself broke loose as seen in this example.
To me, this is a case of exceedingly dumb weight savings. How many grams are you really going to shave out of a seatpost clamp? Sure, the counterargument is that if you shave those few grams out of every part, then the savings really add up. But if you've pushed the envelope a tiny bit too far on this one simple part with one simple task, it fails, and you're kinda stuck (unless you like riding standing up all the time).
A short digression for a couple curiosities: The tandem was creaking before this thing failed, so I wonder if it was starting to crack before it blew up completely. To find out, I'd need to break it all the way through to see if the inside of the crack has the telltale polished surface of metal that's been rubbing together for a while. Also, the stamped size on it is 32.0, while the calipers tell me the seat tube is 31.8mm. I couldn't get a good reading on the busted clamp to know if it was actually oversized, but I have to wonder if tightening down a clamp that's 0.2mm too big could fatigue it and eventually cause it to crack.
That's head-scratching theoretical stuff for another day, though, since I decided to replace the busted clamp with a big honkin' honest-to-goodness 31.8mm Surly Constrictor:
Lots more meat on that baby. And a bigger bolt to generate a load of seatpost-crushing torque. Heavier? Well, sure. But if it keeps the bike on the road, who cares?
And since the tandem featured those dumb Cannondale clamps front and rear, I splurged and got one for the missus too: