EC parts were not (until later) a separate mask.
When chips are made a certain number of them are faulty.
To find the faulty chips they all go on a tester which runs some test vectors (a bit like programs). Not only can they discover faults, but they can tell where in the chip the fault is.
If a fault is discovered but it is say limited to the MMU, or FPU and the rest of the CPU is fine, rather than being binned it is separated and branded EC or LC and sold to a different market (Washing machines etc). This can seriously increase a semiconductor's yeild (the percentage of viable chips) and thus profits.
Often demand for EC or LC parts would outstrip the number of faulty parts being found and in that situation Motorola would sell fully working ones as EC instead.
It is a lottery as to whether an EC part will have a working MMU.
Later to save money I *think* they made specific LC and EC parts which physically didnt have an MMU or FPU and had a smaller die.
Even then if stocks of full 68060's were running too high, rather than make some more EC/LC chips, they would sell rebranded full ones if it saved money.
I hope that explains why your EC part has an MMU.
