That's not base58, that's an xprv (a master private key)
HD wallets use an xprv to derive other private keys from...
You either missed, and copied the wrong key, or you have a bunch op master private keys (in case they all start with xprv and have a similar lenght to this one).
Either way, it's not that hard to derive the first x private keys/public keys/addresses from each xprv in your list by using a small python script... You can then import those private keys directly into electrum or core...
The only thing is: you'll need to know the correct derivation path we have to use to derive the private keys from the master private key...
These are BIP32 keys. There is an open source site called bip32.org that formats these keys to a normal WIF format.
ike i said in my post: they're master private keys, afaik, that's just a synonym of a BIP32 Key.
BIP32 is the BIP where they described key derivation...
Even if the OP uses bip32.org, he still needs to know the derivation path, and he still has to import those private keys derived from his 400 xprv's by hand... Like i said in my previous post: a small python script could saved him hours/days of work... The only thing i have to know is the derivation path and wether or not he messed up and accidentally pasted a master private key instead of something else...
