I like curb signs and have three different versions of the Mobil ones.
Garage-A-GoGo yes you do have the wrong base on your curb sign. You should have the large base that could be bolted or secured down. These larger signs would blow over due to the larger surface area during a wind gust. The larger base is seen with the Keyhole version.
I'll attach photo's of my signs & bases.
Dave's Garage & Memorabilia, Inc.