2031/1974 Nepal 25 Rupee 60% Silver 17 Grams

Thanks, but do you have this coin?

I should have mentioned this, but there is a note for KM#838 that the thickness is 2mm.
This is also mentioned on the Numista page http://en.numista.com/catalogue/pieces32395.html

The diameter according to numista is 36.2mm.
If you calculate the weight of a 50% silver/copper cylinder with a diameter of 36.2mm and height 2mm, you get just 19.88g!
An actual coin should be lighter than this, assuming the thickness is measured at the highest point, the rim.
Aside from some unusual medal-like coins, it is impossible for any 50%/36.2mm/2mm coin to weigh 25.6g.

There is proof version of this coin, KM#838a, with a note: 3mm thick.
If you do this calculation for KM#838a (proof, .925 silver, 28.28g, 3mm thick) assuming the same diameter 36.2mm, you get 31.99g.

This is normal, the value is higher than, and close to, the listed value of 28.28g.

The specs of the coin in the picture are:
Weight 17.10g
Diameter 36.0mm
Thickness at rim 2.1mm

BTW the weight of a 60%/36mm/2.1mm cylinder is 20.99g, still well under 25.6g.

The average thickness of a coin
(not a cylinder) of 60% Ag/40% Cu, which is 36m diameter and a mass of 17 grams,
is 1.70mm

Given that the rim is 2.1mm thickness, the guesstimate for the thickness between the obverse and reverse field interfaces, would be around 1.5mm, or perhaps a bit less.
That can be checked with micrometer screw gauge for confirmation. This should be done in at least 6 places, and averaged. The instrument needs to have point jaws, not flat jaws.
Often with coin design, the fields can be slightly dished to allow for the thickness of the raised parts of the design. With this particular design (both sides), I believe not much 'dishing' would be needed.

If correct, the depth from rim to field on each side should be around (2.1 - 1.5) divided by 2 = 0.3mm.

To my way of thinking, a mass of 17 grams would be reasonable.