Just for the benefit of the non-Dutch speakers (courtesy of Google Translate)...
The cross beam for the front passenger seat .... or the crossbar behind the front passenger seat. Keep in mind that it may have at some time been converted from RHD to LHD
Hi Stefan, thank you so much for your tips, it's an RHD, I'll dive in tomorrow....
Found !, I have an RHD on the crossbar behind / under the driver's seat. Tomorrow to the RDW for NL registration.
Good news, glad you got it.
There ought to be a plate in the engine compartment too, as below, and you may find the last 4 digits of the engine number stamped on the bonnet hinges.
There is an earlier thread here: Location of Chassis Number