Hi Phascogale,
Sorry I can't help you with your specific query re access to the inner enclosures of the Deer & Cattle sheds but I have a feeling that the eastern corridor was cut off much earlier than the western side.
I was first taken to London Zoo around 1958, and I remember the overwhelming sense of awe in viewing the rhinos' in their inner enclosure separated by only a massive glass panel. The rhinos appeared to me to be absolutely huge. There were black and white but at that age I could not tell the difference. The other side of the house contained all manner of deer including red deer, which I cannot remember seeing at Regent's Park since then.
By the early sixties the Cotton Terraces were developed, and the deer moved there. That's when the eastern side was given over to dogs, and I cannot remember ever being able to view the inner enclosures from that time onwards.
Hope that's accurate information, but time plays tricks on the memory.
You never knew what you were going to see in the D & C. There were a few internal dens not really viewable by the public. One day craning my neck into one of these I was delighted to see a Gnu, enabling me to "spot it" in my "I-spy at the Zoo"!
Best wishes,
Michael