Thank you Richard for your research. Now you do highlight the fact that the 3 countries came together (Belgium, Netherlands and Luxembourg) as a group called Benelux. I think that is sufficient to show it was part of the 4 Heads.
The following is from Wikipedia -
The
Benelux Union (
Dutch:
Benelux Unie;
[4] French:
Union Benelux;
[5] Luxembourgish:
Benelux-Unioun),
[6] also known as simply
Benelux, is a
politico-
economic union and formal international intergovernmental cooperation of three neighbouring states in western
Europe:
Belgium, the
Netherlands, and
Luxembourg.
[7] The name
Benelux is a
portmanteau formed from joining the first few letters of each country's name –
Belgium,
Netherlands,
Luxembourg – and was first used to name the customs agreement that initiated the union (signed in 1944).
[8] It is now used more generally to refer to the geographic, economic, and cultural grouping of the three countries.
Cooperation among the governments of Belgium, the Netherlands, and Luxembourg has been a firmly established practice since the introduction of a
customs union in 1944 which became operative in 1948 as the
Benelux Customs Union. (end of quote)
Now a leader of a nation is often referred to as a `head.` So too those who run organisations. It refers to leaders.