Genuine Brand New Single SBN Mikuni Carbs can be found for about $150 bucks on eBay. But that's just for ONE Carb.
The Knock-Off Carbs are made from inferior castings and the tolerances will not be right.
Look for terms like "Genuine". The Genuine Stuff comes with Mikuni Decals.
Contact Mikuni USA and ask what to look for in a Mikuni Carb to verify it's authenticity.
They will be very happy to help you since they don't want knock-offs stealing their business and damaging their Brand reputation.