Actually, according to the ZSL Annual Report for 1955, the Zoo Quest Expedition to Guiana brought back two West Indian manatees for the zoo.
The ZSL Annual Report for 1955 has a section on the Zoo Quest Expedition to British Guiana. According to this, the expedition collected twenty-three mammals, one hundred and twenty-six birds, twenty-two reptiles, eight amphibians, two fish and over two hundred invertebrates.
In addition to the manatees, a few animals are specifically mentioned including three giant anteaters, a tamandua, a capybara, a young Brazilian tapir, some capuchin monkeys, a spotted cavy, a rat-tailed opossum, a cock-of-the-rock, two hoatzins, some humming birds, an anaconda, a matamata terrapin and two electric eels.
The Annual Report's appendix lists all the mammals, birds, reptiles and amphibians (but not the fish and invertebrates) that the zoo obtained during the year. Unfortunately, though, it doesn’t identify what specimens were acquired as a result of the Zoo Quest Expedition so it's difficult to get an accurate list of the species received from that expedition.