What we know
Post office 1876-1877 and 1882-1888 on Big Sandy Creek; Scott labels it 'Big Sandy 1876 (Later Calhan 1881)', so it is Calhan's predecessor.
Where it was
'Big Sandy 1876 (later Calhan 1881)': Scott, Historic Trail Map of the Denver 1x2 Quadrangle (USGS I-2639, 1999, S-0106), symbol read from the reprojected sheet (good to a few hundred feet). Place Names of Colorado: Sec. 13, T11S R62W, 'Rural Post Office 1876-1877 & 1882-1888. 30 mi SE of Elbert.'; this point is 0.53 mile from that section's center.
Read from Glenn R. Scott's USGS historic trail map of 1999 and confirmed by a second, independent source.
Sources
Sources are named in the text above. Numbers such as S-0080 refer to our source list. Most newspaper quotations were read in the Colorado Historic Newspapers Collection. Long quotations from recent books are shortened here.