But relevantly, the EEC laid the foundations for the customs union well before it transitioned to becoming the EU, which is what GP is interested in. The EU customs union is separate from the borderless arrangements of the Schengen Area - the UK is part of the customs union but still has border checks - and of course back in the EEC days it wasn't as complete as the EU customs union is now.

Of course Gibraltar joined the EEC back when the UK did...