Miche Fixed Track Sprocket Lockring

+1, Miche = Campag. Not Shimano.

The stock miche lockring will work, it's not dangerous or anything. But it wont last forever if your changing cogs often.

Decent quality lockrings for those hubs are made by Campag (obviously) and EAI. The EAI one is only about £9 is stocked by Hubjub.

+1 Miche = Campag (for lockring threading anyway)

Condor sell an off-brand lockring of this type for £7. Had it for 4 months hasn't needed tightening once.