What does it mean to say an etching or lithograph is “plate signed”?
When an artist scratches their name into the metal etching plate or uses a lithograph crayon to sign their name onto a lithograph plate, the prints that come from those plates are said to be “plate signed”. “Plate signed” and “signed in the plate” mean the same thing. (The term “plate signed” is in contrast to “hand signed”, where an artist signs each print in pencil, crayon, ink, or some other medium. Hand signed prints are usually numbered.)
When an artist places their signature on an etching or lithograph plate, they often write backwards so that the prints have their signature in the correct orientation. Artists also occasionally sign forwards, which means the prints have the signatures in reverse. There are even methods such as using lithographic transfer paper to allow an artist to sign forwards and still get a signature on the print that reads in the proper direction. All are acceptable forms for generating works that are “plate signed”.
Now for some cautions. Just because a seller advertises a print as “plate signed” does not mean that it is. This misrepresentation can frequently be found in ads on eBay, for example. Reproductions on paper done on a printing press of famous oil paintings or original etchings/lithographs are often sold as “plate signed” by dishonest or sincerely mistaken parties. If an entire work is a printed reproduction of an original oil painting, the signature should not be called “plate signed”. (Also, the work should not be called “by” the artist, but rather “after” the artist.)
Also, the estates of dead artists sometimes include a signature to reproductions of an artist’s work that they never signed while they were alive. The original for the signature comes from something else that the artist did actually sign. “Estate authorized facsimile signature” would typically be the correct term in this case.
Consulting a reference work on the artist’s graphic works is a good way to educate yourself before you make a purchase.