UPDATE - NOW SOLD     
set of 8 chrome exhaust barrel nuts with allen key. Purchased many years ago from CB Performance in California. Still in original unopened packet. Item number 3677. Copied description from their website :

The super trick way to bolt on large diameter exhaust flanges. The Exhaust Barrel Nut has 8mm threads in one end and a hex socket at the driver end. Screw it on and tighten it down with an allen wrench (included). You don't have to worry about wrench clearance and the round Exhaust Barrel Nut doesn't dig into the exhaust flange as with standard hex nuts. Installation with Exhaust Barrel Nuts is easy, fast and efficient. Exhaust Barrel Nuts are plated and add a touch of class to your engine. Use 'em to bolt on the alternator stand, fuel pump and wherever you want to clean up and customize your engine. Just to make it even better, we include a heat treated allen wrench with each set of eight Exhaust Barrel Nuts.

Similar item (nuts only sold)  at Coolair.co.uk,  Part Number: AC000805 (£2.05 per nut)

£8.00 for the set including postage. Payment via Paypal preferred.