It's a black, phillips head bolt or scr*w near the top of seat post. One in front & corresponding one rear. Function???

it probably holds, or prevents rotation of the clamp/head on the post.

My educated opinion (mechanical engineer) is that [MENTION=305894]oddjob2[/MENTION] almost said it right. It absolutely does do both functions, radial positioning and retention of the head.

Prevents the chair from rotating when pumping hard in too high a gear and keeps the chair on the bike when the motor leaps up to climb a hill (crating a negative pressure are above the saddle)!

yes. 2 Component parts.. Head is one piece , the Tube -shaft , the rest .

Alright then. I thought that might be it, but I thought seat posts are usually all one piece. Modern ones I've seen, like Kalloy or Nitto don't have those, nor do any of my other vintage bikes. Good to know. Thanks everyone.

It's the anti ejection set screw I believe. Back it out and your in orbit.

I can't tell from the photo, so: does it appear to be original, that is, done at the factory? I rather doubt it. My suspicion is that whatever the factory did, in intending to keep the two parts of the post together, it has failed. Rotation is a symptom of the problem, but is not the actual problem. The through-bolt solves the symptom, but not the problem. I'm not sure the post is safe to ride.

unscrew them and see what happens.

Oval head machine screw, given how well the head fits in the countersink in the outer tube I'd say definitely factory installed, not a backyard patch job. Very likely a "budget" seat post, I doubt it would ever fail but I'd probably replace it just because it would bug me.

Either factory or a skilled amateur.

I love it when the springs and washers pop out.

It's a factory screw, I have the same seat post but I'm not at home to verify manufacturer. I'm guessing SR or Kalloy. I can double check tomorrow if someone wants to know.

Makes me wonder what holds the seat posts together that have a carbon fibre mast joined to a cast aluminum head. I'm guessing epoxy is used.

The seat post clamp is Kalloy so I suspect the post is also, although there's a symbol but no brand on the seat post. This was definitely​ done at the factory - professional job. It doesn't turn as far as I can tell, so I'm leaving as is.

Phillips screws are bush league, factory or not.

So what you're saying is, that if it's not adjusted just so, I could be in orbit?! That could be very interesting. I'll make sure my helmet is on extra tight.