Information if it is testnet or not is not included in private key itself. Only in prefix which is used to generate WIF.
2. Looking at
https://docs.blockstack.org/understand-stacks/accounts, especially example of their private key when generating account also 66 characters. Although i don't know why their example use prefix
03 (maybe it's standard for Stacks 2.0 network)
npx -q stacks-gen sk --testnet
{
"phrase": "guide air pet hat friend anchor harvest dog depart matter deny awkward sign almost speak short dragon rare private fame depart elevator snake chef",
"private": "0351764dc07ee1ad038ff49c0e020799f0a350dd0769017ea09460e150a6401901",
"public": "022d82baea2d041ac281bebafab11571f45db4f163a9e3f8640b1c804a4ac6f662",
"stacks": "ST16JQQNQXVNGR8RZ1D52TMH5MFHTXVPHRV6YE19C",
"stacking": "{ hashbytes: 0x4d2bdeb7eeeb0c231f0b4a2d5225a3e3aeeed1c6, version: 0x00 }",
"btc": "mnYzsxxW271GkmyMnRfiopEkaEpeqLtDy8",
"wif": "cMh9kwaCEttgTQYkyMUYQVbdm5ZarZdBHErcq7mXUChXXCo7CFEh"
}
In this example the private key is 0351764dc07ee1ad038ff49c0e020799f0a350dd0769017ea09460e150a64019.
01 at the end is a flag to indicate to use compressed address.
Because the program was launched with --testnet the generated WIF is "cMh9"... - which is created by adding "ef" at the beginning.
This same private key (with
compressed flag) for the real network will produce WIF
KwLAJ2aLoqCRHy5VawfR3B6a8rGBC7XVDCi9ihK1y63XGTgP3VbR