My reply; photo of that trip on P.96 of John Armstrong’s Locomotives in the Tropics 3 http://www.railshop.com.au/locomotives-in-the-tropics-3-john-armstrong-book
M636C wrote; Even earlier, I remember a tour to Toowoomba with 2100. Not only was the fuel limited, they had removed the dynamic brake unit and replaced it with a blank cover painted blue.
My reply; Any chance of checking the date of that Toowoomba trip, please? P.98 of John Armstrong’s Locomotives in the Tropics 3, mentions the first few delivered without dynamic brakes due to a delivery problem from the USA. First trial trip up Toowoomba Range 10 December 1970 with 656 Express Freight. Dynamic brake retro fits from March next year.
Edit; Also, does your tour trip to Toowoomba explain the picture on P.73 of your 1973 red "An Australian Diesel Locomotice Pocketbook" with a blank cover over the dynamic brake area?