1865 Half Dime Proof Or MS, How To Know?

Is the reeding sharp or dull?

The same difficulty (MS or Proof) occurs with other coins.

This is how I understand the 1856 Flying Eagle cent is treated.
Rick Snow's attribution guide had it worked out.
But as I understand it the TPG's all had their own opinions.
PCGS used to go by Snow's definitions.
But they threw in the towel and did away with Snow's definitions after years of varying opinions.
Only a Proof grade is now assigned by PCGS to any 1856 Flying Eagle cents.

I believe NGC has always graded them only as Proof.
I don't know about ANACS and ICG.

And I have a copy (had to finish breakfast and go upstairs to my office)...

All Half Dimes for 1865 - 13,000 business strikes and 500 proofs were struck on a single day, April 21st.

A single die pair is certainly possible.

That forces you back to die wear characteristics.

https://www.PCGS.com/coinfacts/coin.../images/4386
https://www.ngccoin.com/coin-explor...865-h10c-ms/

Look at PCGS' MS67+ Plate coin... it shows a clear clash on the reverse to the right of the E in dime. The untoned 2nd example has it but less distinct. The third coin does not. NGC's plate coin has a more detailed clash - the fold of the drapery is present too.

Look at the dentils below the date all the way over to Liberty's foot. Looks like a die crack?

That gives you the start of an emission sequence.

Start haunting the auction archives...

https://coins.ha.com/itm/seated-hal...type=NGC1229
https://coins.ha.com/itm/seated-hal...type=NGC1257

PS: same coin, different holders (MS67 CAC in 2015, MS68 in 2017), no clash

Eventually, you will be able to say "this is the earliest known die state".

If you coin matches that it's possibly proof. If it's from a later die state, it's absolutely not.

No clash...