I think Alex is right about the 3/9 referring to the needles.

I know Transcontinental Oil entered several ventures with other oil companies. I have a photo of a "Marathon" bulk truck from the same era with Skelly signage and have seen mention of them dealing with Skelly.

I would guess they had an association with Bluebonnet in some area where this may have been put out.

Thanks for the info.
Chuck