I figure the shape would have been round (P3 style eyewear) or octagonal for a bit longer until someone solved the need for greater downward coverage (which the B&L aviator did). Most specs and tinted glasses/sunglasses (amber, black, brown, blue, green) in the 19th Century tended to be round in shape. The 1950s were really the decade when sunglasses started taking on different shapes, intro to cat eyes,more squared lenses, etc.
So my guess is that round sunglasses will be the dominant fashion shape for metal sunglasses in lieu of aviators,
I bought the following book a decade ago, and it has some marvellous pics of eyewear from the 19th C. and beyond:
One cool piece from the 18th C. in the book is this over-the-head specs: