Did the 15" Texaco lubester sign ever come with a black back?

Generally, no. However, Texaco made the 15" utility sign for so long; there is a possibility of a black back depending on the era of the sign (REG U.S. PAT OFF ( black T), REG T.M. (black T), REG T.M. (white T), or Circle R).

It was black T, holes are 12, 3, 6, 9
Reg US Pat Off
here photo's showing that the holes did come in both patterns.

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

Here are pics of sign in question

The black and white lubester photos are cool because they show the difference between the static display visible oil tubes with the small porcelain signs Vs. the true visi tubes with the small cast signs. Thanks for sharing.

Not to change the subject, but noticed the Texalene Benzol Blend sign....I have never seen that before....another sign to add to the bucket list...LOL