Yipes! 

The only part that comes to mind that is similarly shaped would be the pivot pins for the paper bail arms, but they're smaller - and there wouldn't be three of them (never mind that you would have noticed the paper bail falling off your carriage). 

Have you ever had the carriage off that SG1 before? Did they fall out as you were lifting the carriage off the machine, or doing something else like turning it over after it was off? Does the carriage have a paper injector?

EDIT: Posted my reply at the same time as Michael did, who thankfully knew what it was and it makes sense how such an item could find it's way into a typewriter carriage. Keep shaking it, Beak. Maybe there's some loose change in there too...

I was advised to look for foreign matter in the machine when I asked someone why the slugs would not quite reach the platen when typing - this was their best guess.  This particular SG1 has had a sad life before I got it, but repairs are now to a state where she's nearly breathing - just can't get those slugs to the paper!  I know the problem is in the machine, not the carriage, because I swapped it with other SG1 carriages of the same pitch and it worked fine on other SG1s, but none of my SG1 carriages will work on this one - the problem is always the same.  The keys rise as normal, and the action feels normal, but the slugs stop dead about a millimetre from the platen - unless I really hammer the keys - then they will.  Any other thoughts on this problem would be kindly received - determined to get this one working!

You might want to look around in the escapement mechanism, or the universal bar frame that trips the escapement mechanism to see if another one of those rivet-looking things is gumming up the works.  You may have quite a load of those little critters infesting your machine.  Only the Good Lord above knows just how many strange things I've fished out of typewriters all these years.

Congratulations on the progress you're making, Beak.

I'm sure you're aware that cylinder adjustments are a very delicate procedure involving micro adjustments to set up the proper gap between the type slug and the surface of the platen. It's the ring, and not the platen that should be stopping the typebar's swing. Arbitrarily moving the carriage could result in the platen taking the full brunt of the type swing, which would be very damaging to the machine, never mind what it would do to the ribbon and paper.

For the SG1 there should be a gap of 0.007 inch (0.18 mm) between the surface of the typeface and the platen, and this should be tested using characters from the middle and each extreme end of the keyboard, and of course with the carriage moved between its extreme right limit of travel, mid-position, and extreme left to make sure that everything is true.