TY  - RPRT
TI  - Letter Circular 657: National Bureau of Standards specification for proving rings for calibrating testing machines (superseding Letter Circular 548)
PY  - 1941
PB  - National Institute of Standards and Technology
CY  - Gaithersburg, MD
SN  - LCIRC 657
DO  - 10.6028/NBS.LCIRC.657
ER  -