I do see a Marietta on eBay currently with an inset heat ring. It is marked. My expectation would be that if yours were Marietta, it would also be marked. If your piece was able to be compared to a marked Marietta and found to be identical in dimensions and in all other ways, that might support them as being the maker. But seeing as copying of others by probably hundreds of small foundries went on in those days, it's highly unlikely. As stated, trying to tie an old unmarked, bottom-gated skillet to a known maker is typically an exercise in futility.