Thank you for the info!
I found a text in my book that stated "whenever the control valve is in neutral position fluid is free to flow between the steering jacks, thus allowing the aircraft to be towed"... but if I understand you correctly that is not true on the 747 where you have to insert a pin to enable towing?
I don't know if you are familiar with different learning books (e.g. Oxford), but are they generalising aircraft systems?