The plate is engraved in zinc, and the stamped number on the upper right refers to its placement in the book - page 131. In the original printing of the book, the illustration was in full color which would have required additional plates, one for each color. This is the black, or key line plate. When the color printing was dropped, in the 1930's, this would have been the only plate used. With the popularity of the Oz books, new printing plates had to be made as old ones wore out, and the publishers kept the original drawings by John R. Neill for this purpose.
Creating the printing plates appears to have involved quite a bit of hand work, in addition to photo-mechanical etching. In this detail, the circular marks of a grinder, used to clean up the background, can be plainly seen.
This has been very nicely presented in a custom box which provides a space for the plate, as well as a new proof pulled from the plate. There is also a pamphlet containing an essay concerning the Oz illustrations, written by Michael Patrick Hearn. It's fascinating as an integral part of the creation of the Oz books!