The original date is a useful variable for three questions about the design. It is silent on the question that decides the risk.
Question 01
Was there a protective earth in the original design?
Older Portuguese residential installations were frequently built with no continuous earthing system connecting the board to an earth electrode.
How it is established: continuity is measured. It cannot be observed.
Question 02
Was there differential protection?
The 30 mA high-sensitivity residual current device is a modern RTIEBT requirement. Earlier boards often carry none, or one low-sensitivity device for the whole installation.
How it is established: verified per circuit, not by presence in the board.
Question 03
Was the circuit count designed for today's load?
An installation laid out for lighting and a few sockets now carries an induction hob, air conditioning, a heat pump and sometimes a vehicle charger.
How it is established: circuits and protection assessed against actual load.
The question age does not answer
Is any of the original installation still there?
A 1970s building tells you what was installed in the 1970s. It tells you nothing about the interventions since. In Lisbon resale stock the installation is usually a composite of several dates under one board, which is why the construction year is not a usable proxy.