They are identical parts from Renault, but a second hand one 'May' not be the same, as the williams runs black bump strips with plack inserts and some 16v runs black bump strips with silver inserts. If you get a 16v bumper you will have to change the inserts if this is the case. You can buy the inserts from Renault.
There are a lot of Williams running around with 16v bumpers on for obvious reasons.