To me it is hard to compare a MS-66 with a G-4
If you could find a picture of a slabbed G-4 online
compare it with that.
One of the markers to look for is between the buffalo's legs.
The die for the 3 legged Buffalo did sit for a while
before being polished and put back into use.
Between the Buffalo's legs the die got some rust build
up. On mid to higher grade nickels this can be seen.
I have never seen lower grade 3 leg buffalo's in hand,
so I don't know if it can be seen. But I would look for it.
If there is a good dealer near you, I would ask for a opinion.
What ever you do don't sell it, before getting it into a
slab.
This really is a good coin to send into ANACS. It is the cheapest way to go, with out spending a lot, and find out
if it is real and what grade.
Later in the year ANACS will have a offer for two free submissions, you just pay shipping.
You have to sign up online, but that does not cost anything.
For example
I sent this coin in

Most were not 100% sure it was real. I sent it to ANACS which cost about $12 to have it slabbed.
I sold it the week I got it back for $80. I bought the
coin for melt. So some good profit.
It I would have sent it into NGC or PCGS it would have
cost closer to $40 to have it slabbed.
Sometimes the profit margin is much better on ANACS slabs.
There are coins like yours, and lower grade keys, that collectors just want to know it is real before they
purchase. Then they might end up breaking it out of the slab
and put the coin in their album. ANACS is very good for this.