Yes, I agree that the world of ancient and modern vintage masks is full of compromises, particularly if the plan is to dive the mask, not just display it.
The elusive top screw
What seems almost impossible to obtain nowadays is a twenty-first-century-manufactured vintage-style mask with a metal rim that can be tightened at the top with a metal screw. Only Chinese companies appear to make such masks nowadays, e.g. the curiously named "Weihai Sunshine Fishing Tackle Company", whose range extends to no less than six different models, one of them coloured green:
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Diving
The curvature
Vintage-style masks came, and still come, in all sorts of curved shapes, even circular:
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Masks
However, we probably consider "classic oval" to be the iconic shape of a vintage-style mask. Be aware, though, that an oval, or ovoid, unlike an ellipse, is an ill-defined shape, so you're going to find a range of mask morphology around, some with flatter tops and bottoms, others approaching circularity.
The skirt material
Most pre-1975 diving masks had natural rubber skirts. Before the mid-1970s a few, quite expensive, masks came with silicone skirts to serve the hypoallergenic market. Later, more and more masks were manufactured with silicone skirts, as much in response to a change in fashion - clear silicone masks were deemed more "photogenic", especially after they were popularised by pretty female Hollywood stars in scuba-based films - as their use by a growing number of people with skin allergies.
Unlike natural rubber, silicone is a synthetic material and I expect Nemrod is right in suggesting that silicone has greater inbuilt longevity than natural rubber. I too mentioned that the rubber of many historical mask skirts will be too hard and perished for the mask to be anything other than a display item. However, everything comes down to compromise, as I said at the beginning. I prefer fresh milk to processed "long-life" milk because it tastes better to me. I have vinyl window sills in my home, but I'd really prefer wooden ones because they look warmer. I choose to wear a more expensive woollen suit instead of a cheaper polyester suit because the former looks better. As you can see from my above preferences, I like natural materials more than I do man-made, synthetic ones. Natural materials have built-in mortality, they are going to decay and fall apart at some stage, like ourselves
. When they are fresh and new, natural materials are "live" materials, with a certain familiar feel, taste and smell. Whenever I entered a dive store more than four decades ago, I knew within seconds from the prevailing odour whether the stock of masks and fins was brand new this month. For me, and this may be highly subjective, silicone, vinyl and other man-made materials are sterile, lifeless substances which may last for ever but pay a price by being without a familiar smell, taste or feel. Marcel Proust wrote a long and celebrated novel which began with a few childhood memories aroused when as an adult he dipped a madeleine cake in his tea and tasted it. The dunking of the cake was a regular childhood treat and tasting it again as an adult recaptured the author's lost youth. Somehow I doubt that some man-made concoction such Cool-aid would have had the same effect. That's why I will always prefer natural-rubber skirted masks to synthetic-silicone ones, although, as a lifelong pluralist, I will also fight to the death for everybody else's right to choose silicone-skirted masks instead.