In fact, the key i found is the one that has been used to send to BTC to the "watching only" wallet and it's not a private one like i though.
It seems you are still very confused as to the differences between private keys, public keys and addresses.
A
private key... (in WIF format) starts with a "5", "L" or "K" and looks something like:
5KMUW3epPCabCcJqXsfBBg5c2bnZ4y2a8Qfo6YLrjoS8UfQy3HB
L41NPChv8z7FxEqjDGbVx7VKc4Y4YWHwDhUETTLeXuwuzmpmcDjw
KyioE9wswGdT46bfx2fwcK2rApY9w91rkyzfjtQiy4eZnmWfGNgX
A
public key... well that's generally just a string of HEX and looks something like:
uncompressed 130 HEX chars using [0-9A-F]:
04A627FBEA459A726926F1956898A67EBA5EC47EA53C1FE0D6977077F162D3FA16A6EE7FE9E46554A756110E4506A0334C9A4E5C1C46643654B191B734350A0F8B
or
compressed 66 HEX chars using [0-9A-F]:
03A627FBEA459A726926F1956898A67EBA5EC47EA53C1FE0D6977077F162D3FA16
Public keys can also be encoded as
Addresses and begin with "1", "3" or "bc1" and will look something like this:
18tW1FHLv21JQtdLCpYSpQUSQiqrbE5Vbo
38fxhsGvmuxxWzDMvjdvKLEZ9JgKSMSVGa
bc1qdpav4z3k095hn9ypt7xfsfnjut7gguz2gtf6gx
Which of the above do you have? private key? public key? or address?