The cards were issued by RKO in 1951 for the '52 re-release. Copyright in lower left corner.
In the lower right, the National Screen Service info plus the re-release number "R-51".
Above images courtesy of the Michael Filippello Collection.
This lobby card scene of the dwarfs discovering Dopey to be the "monster" from upstairs only appeared in the '52 set and no others.
Lobby card set via Heritage Auctions.
The set is seen on the back cover of 1952 pressbook, though a darker color must have originally been planned for the yellow banner across the top.
Pressbook image courtesy of the Michael Filippello Collection.