“Ebbett's Pass” or “Ebbetts Pass?”

The Ebbett's Pass Trailhead is a gateway for hikers and horseback riders to the Pacific Crest Trail, connecting with scenic lakes as well as ancient volcanic peaks and rock formations on both sides of the Pacific Crest, including the Carson Iceberg Wilderness. The well-posted sign along California State Highway 4 in Alpine County, California, directs visitors to this trailhead and clearly says “Ebbett's Pass” (see picture). A historical landmark board uses the same writing and explains that this pass is named after Major John Ebbett, who, in 1853, suggested this location to surveyor George H. Goddard as a promising route for the Transcontinental Railroad [1].

Referring to the family name Ebbett, “Ebbett's Pass” is a correctly written possessive phrase. But why do we find the form “Ebbetts Pass” in so many documents?

Apparently, authors didn't simply got tired of including the possessive-indicating apostrophe. Instead, they refer to “Captain” John Ebbetts [2]:

It wasn't until 1850 when John Ebbetts—Captain of the Knickerbocker Exploring Party of New York—crossed this pass with a large train of mules, guiding a party of miners into the then gold-frenzied California.

A few years later, John Ebbett, let's call him John Ebbetts from now on, led a survey party for the Atlantic and Pacific Railroad Company to this high mountain pass in search of a possible route for the Transcontinental Railroad. His friend and lead surveyor George H. Goddard eventually named the pass Ebbetts Pass in honor of the “Knickerbocker pioneer” [2,3].

Referring to the surname Ebbetts, the genitive case seems to be dismissed: I haven't seen the writing “Ebbetts' Pass” or, worse, “Ebbetts's Pass.”

Keywords: grammar, spelling, writing, name places, history.

References and more to explore
[1] Wikipedia: Historical Landmark: Ebbett's Pass.JPG [http://en.wikipedia.org/wiki/File:R39287918565_Sign_-_Historical_Landmark%EF%BC%9A_Ebbett%27s_Pass.JPG].
[2] Sierra Nevada Geotourism MapGuide: Ebbetts Pass National Byway [www.sierranevadageotourism.org/content_detail.php?uid=sie6DA429BAE628F80E3].
[3] Judith Marvin: Ebbetts Pass History [www.calaverashistory.org/overview/ebbetts-pass-history].