The 1942-D over horizontal D Jefferson Nickel is a rare repunched mint mark, which is extremely elusive in high grades. This variety is easily identified without strong magnification. As implied by the name, the first “D” is punched horizontally with the curved side facing down with the second “D” punched in the regular alignment.
Only a small number of examples were discovered in the late 1970’s, with few coming to light since. This is considered to be the rarest of the major Jefferson Nickel varieties in mint state, and only a handful of pieces have graded as such at the major grading services. Examples graded MS64 with Full Steps have brought five figure prices at auction, setting records for any Jefferson Nickel.
taken from: http://jeffersonnickel.org/