Schwarze Electric Company
The MB/GPW used either a Sparton (Sparks-Withington) or Schwarze (Schwarze Electric Company) horn. In preparing this page I was unable to unearth any information on the Schwarze company. So the focus for now will be on the Sparks-Withington Company.
The Withington Company was founded in 1900 in Jackson, Michigan. They started with 15 employees and a capitalization of $15,000. William Sparks joined the company and shortly thereafter the company was known as the Sparks-Withington Company. In 1911, the company developed the first electric horn in the US. The trade name adopted was Sparton. During World War II, Sparks-Withington made many different war products, including the Sparton horn for the jeep. The company won five Army/Navy "E" Awards, one for each plant. For more information on Sparks-Withington, now known as Sparton Corporation, see their website.
Richard Grace has supplied the following pictures and measurements. The black horn is the Sparton and the red primered horn is the Schwarze. Richard posted (on www.g503.com) the length of the trumpets as 6-1/2 inches for the Schwarze and 6-15/16 inches for the Sparton. He has since discovered variations in the Spartons. The two black horns are both Spartons, both marked identically, but one has the 6-15/16 inch trumpet and the other measures 7-3/16 inches, a full 1/4-inch longer. There is a slight difference in the back plate as one has a rectangular slot and the other doesn't.
Hopefully, this will help you be able to better identify the correct horn for your MB/GPW and not spend excessive amounts of money for an incorrect horn.
During the early 1980s, Willys Minneapolis sold 6v and 12v versions of the Sparton horn. These were reproductions marked Sparton on the back plate. I'm not sure if the tube was also marked. Beachwood Canvas has a reproduction (in both 6 and 12 volt) in their catalog but I couldn't find pricing so they may not actually have them currently available. Or you can try Richard Grace.