"During the late 1950s another area of progress was the development and deployment of AN/FPS-14 and AN/FPS-18 gap-filler radars. Having a range of around sixty-five miles, these radars were placed in areas where it was thought enemy aircraft could fly low to avoid detection by the longer-range radars of the permanent and mobile radar networks. Gap-filler radar deployment peaked in December 1960 at 131 sites throughout the continental United States. Because the introduction of gap-filler radars alleviated the need for civilians to scan the skies for enemy bombers, the ADC disestablished the Ground Observer Corps on January 31, 1959."

Searching The Skies
USAF Air Combat Command
June, 1997

AN/FPS-18 site layout

AN/FPS-74

Frequency-diverse gap-filler radar, similar to AN/FPS-63; neither was ever fielded.

A magazine advertisement from Jan, 1962 "Time" illustrates what is believed to be the AN/FPS-74

In the USGS aerial image above, from the "TerraServer" website, http://terraserver-usa.com/image.aspx?T=4&S=8&Z=18&X=5969&Y=86369&W=2, the old Air Force gap-filler radar radome is the round white object immediately to the left (east) of the baseball diamond. The Nike earthen berm is the brownish, tree-covered area around it to the east and south.

Here's an excerpt from a newspaper article about Montana gap-filler installations.

Here's a news clipping on gap-filler diesels

Adding a radome to a gap-filler