TY  - RPRT
TI  - Letter Circular 548: specification for proving rings for calibrating testing machines (superseding Letter Circular 409)
PY  - 1939
PB  - National Institute of Standards and Technology
CY  - Gaithersburg, MD
SN  - LCIRC 548
DO  - 10.6028/NBS.LCIRC.548
ER  -