Here are mine. I struggled with the Sunoco bath signs for a long time questioning authenticity at every turn due to inconsistency in the images. Comparing many signs to each other and trusting none of them.
Finally an oldgasser who I just trusted along with his story on the sign(s) lead me to believe and buy.
I got these along with a main bath sign and a photo of the actual station they come off of. The story, along with the paint drippings, and my trusting the seller leads me to trust that mine are authentic. And they have black backs. But that is in no way to say that other, fully authentic signs, didn't have yellow backs.
After my Standard Gasoline globe thread,http://www.oldgas.com/forum/ubbthreads.php?ubb=showflat&Number=684089&page=1
consistency I guess can't always be trusted. Different markets, different manufacturers...
Having said that, and with this guys "other listings", and the poor quality of the images on the signs in question, I would think they are fake as Bob and Jim stated. But I wouldn't say it's because of the black backs.
P.S., one thing I really like about mine as well, is the quality of the images, the sharpness in the ruffles of her dress, the air gap under both the woman's and the mans arms, the better detail in the mans shoes... But again, I just think there were at least 2, if not more companies that made these signs for Sunoco.
What's strange to me is we nit pic signs to death on this site calling out certain ones as fake, due to this little thing or that little thing. Yet it turns out that inconsistency may just be part of the real world game. I don't know. This hobby is sometimes hard to trust across the board.