This is one of two 1876 transitional double eagles. This design has the obverse style of 1877 with the top of Liberty's diadem centered between stars six and seven combined with the regular reverse design of 1876 having 'Twenty D.'. The obverses prior to 1877 have Liberty's coronet virtually touching star seven.
This design in gold is unique and first came to light in the June 1909 Numismatist when J. Haseltine sold the piece to William Woodin. It is unclear whether the piece was originally owned by Idler or Snowden although we suspect the latter. Later owners include Newcomer, Col Green and Farouk. It has not been seen since.
The coin has been described as a one-sided proof.
This design was also struck in copper J1489/P1642 with only the following 2 confirmed.
1) Byron Reed-Durham Museum
2) RARCOA 73 FUN, Paramount 5/76, Bass Foundation
Note: The Judd Illustrated History of U.S. Coins lists one but the image is of a J1491/P1644.
This is a composite using the obverse of J1490/P1643. The true pattern was struck from a different obverse die with a slightly higher date. Photo used are from the Trompeter gold proof collection and are courtesy of Heritage.