Typewriters bugged by the Soviets in the U.S. Embassy in Moscow had been shipped through "normal channels" instead of receiving special diplomatic handling, allowing electronic devices to be placed in them, intelligence sources said yesterday.

They speculated that the bugs were "passive" devices that reflected signals to low-level transmitters hidden in embassy walls, presumably allowing the Soviets to monitor what was written on the typewriters.

Administration officials confirmed this week that the Soviets had penetrated security at the embassy for at least a year, and perhaps longer, by "lifting things off typewriters." They said the bugging was ended after being discovered last year.

One source, who asked not to be identified, said yesterday that the Soviets probably did not gather much valuable U.S. intelligence by bugging the typewriters because little of the most highly sensitive material would be kept in the Moscow embassy.

In addition, this source said, material kept there is handled carefully in special areas that have been "swept" for listening devices, making it impossible for the typewriter bugs to work.

"I don't regard it as terribly serious," the source said of the bugging, suggesting that the security breach is not as threatening to the United States as other intelligence losses to the Soviets in recent years.

An administration official said, "The trouble is that you never know exactly what they did get" with the devices in typewriters. The incident "was taken seriously" by U.S. officials, he said.