This benchmark disk is monumented at the northeast corner of the Sun River Bridge, in the top of the step, or curb, at the bottom of the concrete corner post. It is on the roadway side of the post about 6 inches above the road surface.
The USGS datasheet indicates that the benchmark is in Lewis & Clark County but, given that it's on the north side of Sun River it must be in Teton County.
Incidentally, we had no idea there was a benchmark on the bridge - we stopped to get pix of the old bridge pilings to the west of this bridge and happened to think to check for a benchmark.
STATION DESCRIPTION
DESCRIBED BY COAST AND GEODETIC SURVEY 1959
DESIGNATION - S 229
PID - ST0007
STATE/COUNTY - MT/LEWIS AND CLARK
COUNTRY - US
USGS QUAD - GILMAN (1987)
4.6 MILES NORTH ALONG STATE HIGHWAY 287 FROM THE POST OFFICE AT
AUGUSTA, IN THE TOP OF THE NORTH END OF THE EAST CONCRETE CURB OF A
CONCRETE BRIDGE OVER NORTH FORK SUN RIVER, 12 1/2 FEET EAST OF THE
CENTER LINE OF THE HIGHWAY, 0.9 FOOT SOUTH OF THE NORTH END OF THE
CURB, AND ABOUT 1/2 FOOT HIGHER THAN THE HIGHWAY.
From the NGS Datasheet