To add to the mystery there is an inconsistency within the ZSL Annual Report for 1962 about the actual number of mountain “beavers” acquired that year.
On page 13 it records that four mountain beavers were received in 1962 whilst on page 49, of the same document, it lists that six mountain beavers were obtained.
These were the first mountain “beavers” at London Zoo; they are listed in the Annual Report as a genus new to the collection.
The 1962 Annual Report says that London Zoo received its first mountain beavers; there were four of them according to the collection report, but six according to the List of animals received. There is a photograph of one of them opposite page 16. Dr. S.E. Dicker donated one of the mountain beavers. The 1963 report says that London Zoo received another mountain beaver.
Yes, as you mentioned, the 1962 Annual Report mentions that one of the mountain beavers was donated by Dr. S. E. Dicker; frustratingly, it doesn't provide the source of the other specimens.
According to the 1963 Annual Report, Dr. Dicker also donated the mountain beaver received that year.