Most of the early signs (like this) I have seen have very heavy shelving and have a crude, white finish on the back. However, here is a completed listing on a 15" sign like you described with a finished back. Texaco used several different sign makers, their standardization was great, but I'm sure variations occurred.

15" Texaco Sign


Collecting Texaco and Havoline Signs, Cans, and Giveaways.