The term "antique" is certainly used in the context of diving gear and it's quite common on Japanese websites featuring vintage gear. However, if you Google with "antique" and "diving" you're more likely to find yourself in the setting of surface air supplied brass helmets as worn by standard divers of yore in bulky canvas suits and lead-soled shoes.
As a lifelong snorkeller, never a scuba diver, I agree with the chronological division between the pre-modern and the modern era as being around the mid-1970s. For me the arrival of "modern times" was signalled by the adoption of silicone-skirted masks by people with no allergies and by the first appearance of "composite" fins with their thermoplastic foot pockets and plastic blades. For me, it's a question of materials: vintage fins and masks must be made from natural or neoprene rubber and never oil derivatives.
I have a feeling we'll end up sticking with the cover-all term "vintage". There is a long thread in the snorkelling/freediving forum where alternatives to the term "snorkelling" are discussed. I think the final consensus there was that a change of name would probably find little currency within or outside the snorkelling community. I think the same would be true of the word "vintage", which is in the title of several diving forums and has the added benefit, through its association with the world of wine, of suggesting quality and good taste in precisely dated and located artefacts. Personally, I'd be much more interested in the identification of historical periods within the "vintage era" of diving. My own special interest lies in the 1950s, the decade when snorkelling and diving grew from a pursuit for the few into a popular pursuit for the many, including the family. I love the simplicity of the gear during the 1950s, the willingness to improvise and to innovate at a time when war-weary nations with little money to spare were eager to embrace the concept of leisure for the masses. I'm not just a historian of a bygone age, however. I'm also a keen geographer researching the diving equipment manufacturers in today's world which still make fins, masks and suits the way they used to be made during the 1950s, 1960s and 1970s. The oldest diving mask in the world, Cressi's "Pinocchio", is still in production almost a half-century since its first appearance on the market. Such "classics" are still with us because there are still a few people around who appreciate timeless designs. I may collect fins and masks from the vintage era, but I prefer snorkelling with fins and masks made in 2010 to pre-1973 specifications. I get the best of both worlds and I don't know whether that makes me vintage or non-vintage. All I know is that I would never go snorkelling with a pair of plastic-bladed fins or a low-volume silicone-skirted mask even if you paid me. I've managed to avoid such modern aberrations for almost four decades now and I have no plans to change.