I will assume your scales are accurate. Check against a coin of known correct weight (eg
Morgan dollar, weight 26.73 grams)
If that is the case, density test (which also needs accurate weight measurement), should prove either:
1. incorrect planchet in correct alloy,
or
2. fake, if not of correct density for the alloy.
CuNi density 8.91 g/cc
stg silver 10 38 g/cc
CuNi KM#47, Krause does not mention weight,
stg silver KM# 47a Krause weight 24.74 grams, ASW 0.7357 oz.
Krause does not note KM#47 as silver plated, but the coin in these pictures appears to be either silver plated or stg silver. Silver plating would have almost negligible weight.
You have some further investigation and measurements to do.