I have 2 signs shown below that were definitely used on vehicles - one was a milk sign used on horse-drawn carts and the other was a truck sign for Associated Transport. They both have a high number of mounting holes which I've always assumed was because they needed to tolerate the extra vibration or wind forces. The milk sign has 24 bolt holes spaced about 6" apart.
But I think it would help if you showed a picture or at least described the subject matter. A lot of the older porcelain signs seemed to be produced with an excessive amount of bolt holes. The 36" x 12" porcelain model tobacco signs, for example, had 10 mounting holes also.