Two very bizarre cases.

I bought two coins on eBay.
Kyat 1853 silver (mine has the perfect weight and diameter, but the thickness mm.2. How can it have the original weight with 1 mm. less? ........Platinum?)
2 - coin, 20 sen 1888 Japan (perfect weight, but almost 1 mm. less in diameter.
Can it be fake, or are there different types?
I'm sure my two coins are silver.
Please resolve my doubt.
Thank you.

pictures would be good. Tolerances can vary quite a bit do the diameters match? check the diameter at different points it will likely differ.

I did, the diameter is everywhere more than 0,5 mm. inferior. Silver is tested. Weight, perfect.
I use an electronic balance, not very sharp, but sharp if over 0,1 gr.

How can you be sure its silver and then ask if it's platinum? The SG is massively different.

What is the specific SG of each? and pictures etc.

I made five tests. Only a few metals are heavier than silver. I mean, cannot be a fake of tungsten, platinum or gold. Every dimension is the same, so must be heavier if it has a smaller volume...

Are your calipers digital? could be your calipers or scales are out.

metal or alloyweight (kg/dm3)metal or alloyweight (kg/dm3)
ordinary steel7.8 - 7.9mercury13.6
stainless steel7.48 - 8antifriction metal9.3 - 10.6
rolled steel7.85white metal7.1
aluminium foil2.7 - 2.75delta metal8.6
melted aluminium2.56 - 2.64molybdenum10.2
nickel silver8.4 - 8.9monel8.36 - 8.84
electrum8.4 - 8.9nickel8.8
silver10.49gold19.25
beryllium1.84casting brass8.4 - 8.7
bronze (8-14% Sn)7.4 - 8.9rolled and drawn brass8.43 - 8.73
bronze phosphorous8.78 - 8.92lead11.34
aluminium bronze (3-10% Al)7.7 - 8.7platinum21.4
lead bronze8.8 - 9.5copper8.93
iron7.85beryllium copper
8.1 - 8.25
cast iron6.8 - 7.8tin7.28
light alloy based on Mg1.76 - 1.87tungsten19.1
light alloy based on Al2.56 - 2.8zinc7.1

Digital, two, with the same result. I have 500 coins, only these two are so odd.