Cates Mill
Lyn Topinka. The Columbia River: A Photographic Journey
Shell Rock, Oregon, Post Office was established April 14, 1873, in the vicinity of Shellrock Mountain. The Post Office was later moved across the Columbia into Washington Territory and re-named "Collins Landing". Briefly, in 1878, a second Shell Rock Post Office was established in Oregon. According to Oregon Geographic Names (McArthur and McArthur, 2003):
Shell Rock post office was in operation again on the Oregon side between May 20 and August 19, 1878, with John Cates postmaster. Cates, of a well-known Wasco County family, operated a sawmill a couple of miles west of the present site of Wyeth. The post office was apparently reestablished to serve people connected with the Cates mill. It was about three miles west of Shellrock Mountain but named for that feature nevertheless.
Lyn Topinka. The Columbia River: A Photographic Journey. http://columbiariverimages.com/Regions/Places/shellrock_mountain.html
I don't have the site of this sawmill and post office down yet, but it was surely along this route and the big clearing at the top of the hill is about 3.15 miles west of Shellrock Mountain, as the crow flies, and 1.94 miles west of the old Wyeth town site along the road. So, this is my pick, but it is just a guess.