Even with its glistening emerald-green glass, the boxy 1960s-era Zalco Building in downtown Silver Spring is hardly noticed by many passersby, let alone thought of as a historic structure.

The very idea makes John Cranston, the building’s engineer, chuckle. “I don’t think George Washington slept here or anything,” he said.

But to Clare Lise Kelly, a historic-preservation planner for Montgomery County, and to other architectural experts, the office building at Georgia Avenue and Cameron Street is a shining example of International style. It’s time, they say, for it and other “mid-century modern” buildings and homes — those with sleek, boxy designs from the 1950s and 1960s — to be considered for historic preservation....