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Revised the note and example on DER-107 to address Pete's comments. See also https://en.wikipedia.org/wiki/Coins_of_the_United_States_dollar#Bullion_coins.
Also discussed issue DER-84, which points to the need to clarify how commodity underlying assets are specified. A commodity instrument may have any number of things as their underlier - indices, prices, volatility of prices, etc. The underlying commodity asset (of which there may be more than one) may be deliverable or strictly a reference asset. The reference asset may be a price, index, volatility, or commodity that can't be delivered, such as some aspect of the weather, as well as a negotiable commodity whereas a deliverable asset must be a negotiable commodity that has is a physical thing that can be delivered to the buyer, such as bushels of corn. Our current model doesn't allow for this distinction and should be augmented accordingly.