Under EU labour law it is necessary to treat equal contracts as equal: i.e. two employees doing the same job on the same contract where one can not be treated 'less favourably' than the other. I once had such a claim against a company where a fellow captain had extra benefits allocated. His contract was a copy/paste of mine, but he had some dubious verbal agreements that gave significant unjustifiable advantages. It was not only against me, but all similar colleagues who were not aware of the set up. I complained & won in that the unjustifiable extras were removed. It was a shame as we were punting for being elevated to the same deal. As it was a dodgy deal in the first place there was only one realistic outcome.
I would question whether it would be permissible to issue differing contracts for line pilots at the same base, doing the same job, but being rewarded differently. Much would depend on who would be prepared to stand up or lie down.