First you have to fly the gold to Switzerland to a refiner and melt the London 400oz bars into COMEX 100oz bars.

New bars get new stamps, serial numbers, etc. It’s a spot where illegal gold can (doesn’t have to) be introduced.

Once you melted London 400oz bars into COMEX 100oz bars, then you fly them to New York.

This process in itself can inflate the COMEX price by the cost of delivery which is folded back into the forward price. When dealing with futures contracts it is important to know that the price is almost always localized based on the cost of delivery. There isn’t one unique global price of gold.