It looks like they are metal, I've seen the plastic spreader boots in the West Marine catalog and I don't think that's what these are.
I can't recall ever encountering metal bulbs at the ends of the spreaders. But I learn new things everyday.
Usually, the shroud is restrained in some manner at the spreader tip. Seizing wire or a special tip that is made for holding the wire captive. Then a boot is often placed over that to keep chafe from occurring when the sail lays against or drags over it.
Can you post a photo?