Contact AWS in Miamai, FL
The Publication is
ANSI/AWS A5.8-92
Specification for Filler Metals for Brazing and Braze Welding
However their tolerances for 1992 are + / - 1% as in 50% silver to be 49% to 51%
In manufacturing with automated equipment this can make a difference and some companies tend to work towards the low silver side.
The stuff we sell is now measured in ppm which should be pretty common from a good supplier.
Tom