Founded in 1941 by Haywood Cirker (1918-2000), Dover is an American publishing company specializing in out-of-print (and commonly public domain) titles of many genres, including classical music scores. Most of their (rather substantial) music collection consists of reprints of scores of public domain works, notably Breitkopf & Härtel, C.F. Peters and other well-known music publishers. Consequently, the vast majority of their reprint collection is in the public domain. During the vast expansion of the firm in the 1950s, Dover also produced a number of LP recordings, one series featuring the pianist Beveridge Webster. This venture was not very successful and the recordings are now long out of print.
Dover reprints are fairly easily recognizable. Google books let you have a look in sample pages of Dover prints, so that you can confirm a suspected reprint. The typographical additions in most Dover prints include the cover pages, the table of contents, titles and page numbers. Reprints of the 1980s are harder to recognize, as they only include a standard bold serif-type font as title, as in this example. Since the 1990s, the title is a more slender serif-type font, and next to the page numbers the title is repeated on each page, as in this example.