I don't understand your question. Pre-fork bitcoins (always in a non-custodial wallet) require splitting to avoid replay attacks.
No existence of pre-fork or post-fork bitcoins. Bitcoin is the first and unique bitcoin and other altcoins fork from bitcoin and try to claim they are real bitcoin.
You must sweep your bitcoin safely to another new address and then you can apply the guide was given by @TheBeardedBaby to claim forks of bitcoin.
How Many Bitcoin Forks Are There? You will be surprised!!!Many forks from bitcoin but you must keep your bitcoin safely first by sweep it. Don't claim for any forks before you sweep your real bitcoin.