...this fake Gulf flange was discussed on Oldgas a while back:

http://oldgas.com/forum/ubbthreads.php?ubb=showflat&Number=302954

...here is another one of these signs on eBay, same as the above:

http://www.ebay.com/itm/GULF-SUPREME-MOTOR-OIL-PORCELAIN-FLANGE-SIGN-30S-/150883448686

...the description - look for the 'red flags':

THE AGE OF THIS SIGN IS UN-KNOWN TO ME, I FOUND THIS IN AN OLD GARAGE MY UNCLE WAS TEARING DOWN ON A SECONDARY STREET RIGHT HERE IN TOWN.. THE SIGN IS SLIGHTLY BOWED AS IT WAS UNDER BASEMENT STEPS WITH TWO CASES OF OLD MOTOR OIL SITTING ATOP OF IT.. THE SIGN HAS BEEN CLEANED / RINSED OFF WITH DISH WASHING LIQUID & WATER, NOTHING MAJOR AS FAR AS CLEANING OR POLISHING. THIS SIGN IS HEAVY, THE PORCELAIN IS SHELVED NICELY, LOOKS PRETTY DARN GOOD ACTUALLY.. SAYS GULF REFINIG COMPANY ACROSS THE BOTTOM, IM ASSUMING THIS IS A 1930S ERA SIGN ? APPROX 18 x 24 INCHES.. LOOKS LIKE IT WAS MOUNTED AT ONE TIME BUT THEY DIDNT OVERTIGHTEN THE FLANGE SCREWS TOO BAD, HOLES SHOW SOME CONTACT WEAR BUT NOT LIKE USUAL WHEN ITS ALL CHIPPED / SEVERLY BROKEN OFF, HAS SOME SCATTERED IMPERFECTIONS BUT FOR THE MOST PART ITS IN REALLY GOOD CONDITION. WE HAVE MORE SIGNS LISTED & WE WILL COMBINE SHIPPING WHEN SAFE TO DO SO, WE HONOR PAYPAL BUT WILL TAKE MAIL IN PAYMENTS AS WELL, PLEASE INQUIRE.. THAKNS FOR LOOKING.
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Looking for better Gulf items: signs, globes, cans and paper - especially porcelain Gulf flanges, and Gulf A-38 & A-62 ad glass...