Chet15, he of Red Eagle News Exchange fame, offered up the following comment in another thread. It's this kind of information that makes Ruger collecting so interesting . . . and also makes his data base so valuable. I offer my profound thanks for this contribution.
"There were no "D" marked flattops."
"Ruger did make 9 "A" prefixed 10" .357's which are essentially duplicate guns, but was so early in the game for Ruger on the "duplicating" dilemma that they hadn't thought of marking them with a "D"....marked them with an "A" instead."
"Two "A" prefixed 10" guns have been reported through the years, but the current whearabouts of each is unknown today. The matching numbers to the "A" guns are also 10" .357's, but of those none have been reported matching the two "A" guns that were reported long ago."
"Chet15"
For those interested, he lists the "A" guns as A31571 thru A31579 in the RENE Reference.
Bob's List has record of A31574, as well as 31579 and A31579.