Of course Russian has the word «тайна». It's the generic term for hidden, classified, protected, secret, or unknown information. And «государственная тайна» would be "state secret".
But the stamp on the maps wasn't just for indicating that they are some sort of state secret. It was for indicating the exact level of secrecy in the Russian classification system.
Those levels are «особой важности» (literally: of special importance), «совершенно секретно» (lit.: completely secret) and «секретно» (lit.: secret). And in US government terminology, they approximately correspond to "top secret", "secret", and "confidential", although the boundaries between the levels are defined somewhat differently.
But the stamp on the maps wasn't just for indicating that they are some sort of state secret. It was for indicating the exact level of secrecy in the Russian classification system.
Those levels are «особой важности» (literally: of special importance), «совершенно секретно» (lit.: completely secret) and «секретно» (lit.: secret). And in US government terminology, they approximately correspond to "top secret", "secret", and "confidential", although the boundaries between the levels are defined somewhat differently.