Presumably the ad is referring to Spruce Avenue, formerly at the end of Rice's Point. I asked Tony Dierckins of Zenith City Press about it and he mentioned the 1922 ordinances of Duluth indicate there were several Spruce streets: 

Spruce Street, changed to 18th Street 
Spruce Street, Bay View No. 2, changed to Zurah Street
Spruce Street, Spirit Lake, changed to 97th Avenue West
Spruce Street, Zenith Park, West Duluth, changed to Osman Street

And ...

"Minnesota and Rice’s Point both were originally platted with east-west roads named for trees, and the distinction was that on Minnesota Point they were called streets while on Rice’s Point they were called avenues," Dierckins noted. He believes the first reference above is for the Spruce Street along Minnesota Avenue on Park Point, and technically should be referred to as 18th Street South.

Dierckins provided the Wellbanks map (above), which shows there was still a remnant of Spruce Avenue on Rice’s Point near its southern end in the 1930s. 

"Note nearby Pine Avenue, which survives today as a remnant that is essentially a circular road around the UPS facility," Tony wrote. "Spruce Avenue is gone."