The last platypus in the US was at the Bronx Zoo in the 1950s.
Tim May posted this info on the earlier thread:
On 28th April 1947, one male (“Cecil”) and two female (“Betty” & “Penelope”) platypuses went on exhibit in Bronx Zoo. “Betty” died of pneumonia on 6th September 1948. The other two both lived much longer, the female "went missing" on 1st August 1957; the male died on 18th September 1957.
Three more platypuses, a male and two females, arrived on 7th June 1958; these did not do well, the longest living of these died on 25th March 1959.