COMEX gold delivery requires bars of 100 troy ounces or three one-kilo bars of at least 995 fineness, cast by an approved refiner and carrying a serial number. London good delivery bars are much larger, around 400 ounces, at 995 minimum fineness.
Provenance is as important as metal content. A bar that leaves the approved vault chain must be re-assayed before it can re-enter, which is why vaulted metal commands a premium over identical metal held privately.
Example: a COMEX gold contract at $2,400 delivers 100 ounces, roughly $240,000 of metal, as a single bar or three kilo bars. A 400-ounce London bar is worth about $960,000 and cannot be delivered against a COMEX contract without recasting.
Related: warehouse-receipt, registered-and-eligible, gc, troy-ounce, deliverable-grade