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. 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.