Has the mechanic who changed the pump not got the old one so you can get the numbers from it?
I would assume that anyone fitting a replacement pump would have fitted one with matching numbers.
Did the dealer give you any other information other than it was the wrong pump?
I'm also wondering what diagnostic machine the mechanic used on your car as Renaults can be awkward regarding diagnostics and not all universal diagnostic machines can communicate with them, maybe the fault wasn't the ABS pump.
That said the ABS pumps on the Lagunas do fail, I suffered this problem myself. I fitted a 2nd hand pump with matching numbers and had no problems, the pump didn't have to be coded to the car to work.
But just because the pump I fitted didn't need coding doesn't mean the pumps aren't coded to the car in some way, I don't know if this is the case or not. Maybe the dealers diagnostic machine is showing this, hence him telling you it's the wrong (different) pump. Maybe my car would show up as having the wrong pump as well if they are coded in some way.
It could be that you have been unlucky and the pump on the car now is dodgy as well as the one it replaced.
If you haven't got the original pump to get the numbers from all I can suggest is that you get another pump from a car the same spec as yours.
I've just looked on
and a pump listed for a 1.8 petrol Laguna has the same numbers on it as the pump on my car so I'm not sure if it makes that much difference. Maybe if the car has Electronic Stability Program (ESP) there is a difference, I'm not sure.