There are two coordinate systems in question:
"CRS 1" = EPSG:4326 (good ol WGS84)
"CRS 2" = EPSG:26710 (UTM NAD27 Zone 10 (specifically, zone 10S but they don't seem to differentiate)
For the coordinate in question, in "CRS 1" = EPSG:4326:
39.3662N, -120.35175E (i.e. 120.35175W) = NW summit of Castle Peak in Nevada County CA USA
Converting to CRS 2 using various programs (rounding to nearest meter):
All three of these have very good agreement (within 10m, and that's mostly rounding error):
QGIS 2.6: 07 28 241 E 43 60 566 N caltopo.com: 07 28 239 E 43 60 561 N Terrain Navigator Pro: 07 28 241 E 43 60 565 N
These two are very far away from the others and from each other:
Locus Pro (android app): 07 28 153 E 43 60 552 N http://twcc.free.fr: 07 28 145 E 43 60 785 N
Note for Locus Pro I added EPSG 26710 into its config file, which is a documented and supported thing to do. After emailing with the developer he confirms it is (or should be) using the correct proj4 code:
EPSG:26710 : +proj=utm +zone=10 +ellps=clrk66 +datum=NAD27 +units=m +no_defs
Which is also what shows up when you view the EPSG info in twcc.
Any idea what I'm missing? Hopefully just something obvious? Bottom line is that I'd like to get accurate conversion from WGS84 to and from UTM NAD27 CONUS zone 10S on the Android.