Encryption was implemented in core v 0.4.  A master key was added that encrypts the private keys.  (The master key itself was encrypted via various mechanisms). The master key unlocks the private keys.

Is a master key in the old JBOK wallet simply a private key (with a different name) ie is it the same format as a private key?  I am trying to scan individual bytes to look for it as this wallet was not born encrypted....OR does it just generate a master key on the fly when you encrypt ?

ie a private key typically starts (back then) as 0x01 0x01 0x04 - are there any telltale signs of an unencrypted master key?