and for the Jerez operation, it is highly likely that the 07xx and 023 batch of squawks are designated to the particular ATC unit in Spain who provide you with a radar service.
Occassionally, at certain atc units, individual squawks are permantly assigned to individual aircraft, this means that instead of seeing just the squawk on the radar screen, ATC will see the aircraft registration or flight number.
This is the same principle as the Orcam scheme, or an ATC unit that has computer gernerated squawks.