Thanks for the replies. I googled reverse proof too and I now know what they look like. As an engineer (and token collector) I'd really like to know how the process works. Maybe they polish the hubs and use to make new proof dies when they are no longer shiny enough?

Proof coins originated as a way for the mintmaster to verify the dies were ready to strike coins. The results were so pretty, that they started making small numbers as gifts for VIP's, congressmen, foreign dignitaries, etc. I can't think of a practical use for a "reverse proof" except to make something different to sell to collectors who want one of everything the mint makes.

The US mint did make matte proofs in the early 20th century. These were frosty over the entire coin design. They did it with sand-blasting or acid. I understand it was done mainly for the incuse design Indian gold coins of the period, which didn't have raised surfaces on the die suitable for polishing as done for most proofs. My take is these were also done primarily to produce something "different" for the collectors and for presentation to VIP's.

Is there a coin collector who has maybe visited the Mint and seen explanations of how reverse proofs are made?