I guess it's a rubber 'cap' instead of a 'plug.' The same type of rubber cap that's on the bleeder nipple on the brake caliper with a molded tether. As already stated.
I did check the oem parts page and that part (11) is still available. It's looks identical to the rubber cap on the brake bleeder nipple but the clutch nipple is longer. The p/n is also different. Guess it's not safe to have the longer nipple on the exposed brake bleeder. Whereas the clutch nipple is tucked underneath the seat. I noticed on the oem parts page, that there's also a tiny o-ring (16) that seats below the clutch nipple. The brake caliper bleeders don't have the o-ring. I don't think it's possible to lose the o-ring, but it might wear out etc.
