[Robert DeLong]

That's a good guess, but why would they have the gray plastic tip still on it?

It's also unlike any upholstery fastener I have ever seen.

They look more like double-headed nails. They are used in temporary structures so that the top head is exposed making it easier to pull out of the structure. Also used in electric fencing, where the electrified wire is wrapped around the nail in between the two nail heads.

The gray cap is to protect yourself from getting jabbed?

[gundog]

Bristles from a pin brush for dogs is stating to seem plausible.

[William Tell]

”That's a good guess, but why would they have the gray plastic tip still on it?”

I’ll bet my recliner weighs more now than when new. The reason, at least in part, would be the number of small gun parts I’ve lost in it over the years.

It may be that the upholsterer dropped things into his workpiece and didn’t remove them. My wife occasionally tips my recliner to do work near it. I’m waiting for the day that I begin to find more parts than I lose.