What do you actually mean by this?
The Bitcoin node/client (predecessor of the reference client Bitcoin Core) started the Bitcoin blockchain and build its first and succeeding blocks.
That very first Bitcoin client already utilizes a "wallet.dat" file where the private keys that can spend the earliest block rewards from mining the first and early blocks (excluding the Genesis Block).
Are you saying that the first and earliest blocks are mined by other software and not what the "First Developer" released?

Regardless, find your private key or the wallet where it's saved and you should be able to spend your bitcoins.
The client is nothing but a handler of those keys, without the actual wallet, it's just a node.

What do you actually mean by this?
The Bitcoin node/client (predecessor of the reference client Bitcoin Core) started the Bitcoin blockchain and build its first and succeeding blocks.
That very first Bitcoin client already utilizes a "wallet.dat" file where the private keys that can spend the earliest block rewards from mining the first and early blocks (excluding the Genesis Block).
Are you saying that the first and earliest blocks are mined by other software and not the "First Developer" released?

Regardless, find your private key or the wallet where it's saved and you should be able to spend your bitcoins.
The client is nothing but a handler of those keys, without the actual wallet, it's just a node.