The two latest additions are #373 Whisenant and #388 Daley:
There's a little bit of a foldover on the Daley (lower left corner) that shows the fragility of these proofs. You can also see the handcut nature of each. Here is the previously displayed Groth for completeness' sake:
Groth now resides in the collection of Friend o'the Archive Al Richter but the above scan came from Mark Rios. Mark had a copy of the 1979 Card Collectors Company ad from The Trader Speaks that sold 32 of these, along with a clutch of 1970 Baseball Cloth Stickers:
All known '57 proofs are on the sales list which covers the highs numbers from that year, which gives a universe of 55 possible subjects. And those prices are INSANE!!!
Topps probably ran proofs like this off every year. The 1957 examples have finished backs but others that are known from 1967 have blank backs. I suspect the finished (with backs) proofs were the last to be run before the press sheets were printed. That '67 link also has a '66 Mantle paper proof shown by the way.
Bob Lemke had a nice post on these a few years ago, quite worth the look here. I have to think most surviving paper proofs were retained by people present when they were printed, likely just the printer's personnel and whoever was there from Topps at the time. It's believed the 67's came from Woody Gelman's collection and given his company was selling the 57's, all known paper proofs probably were the result of Woody's hoarding. Woody passed away in 1978 so it looks like his personal copies of the proofs (and cloth stickers) were being sold off.