Just read this wiki article on Tyura-tam, the actual location of the Cosmodrome. It says:
In the mid 1950s, the Soviet Union announced that space activities were being conducted from the Baykonur Cosmodrome, which was assumed to be near the city of Baykonur, in the Kazakh SSR. In reality, the launch facilities were located 400 kilometres (250 mi) to the southwest at Tyuratam to obfuscate the west as to its true location.
I checked the cite and it says the same thing about mid-50's announcment as "Baykonur".
Now, I've read many times that after Yuri Gagarin flew on Vostok 1 (1961), Soviet officials filled out a FAI report to claim an official record, and listed his launch site as Baikonur. So I assumed that's when the launch site was officially named.
But the wiki article and source say it was actually announced in the mid-50's. Can anyone confirm this? I've been hunting/googling for a TASS archive for a while now, but cannot find one.