One of the many things I know little about is tires but I remember tubeless Dunlop fabric radials (nylon?) were OEM on my Brand Lotus wheels when I bought my car. I know of no problems running tubeless nor for not having a second safety flange. Of course, tire construction has changed a lot since 1975 and as JB reminds us YMMV. I do remember talk back then that the steel belted radials that were just becoming popular back then were not supposed to work as well as the fabric radials but if that ever really was an issue, it is not as serious a situation now although in a conversation with Ken Gray at DBE, he suggested that new tires have some handling deficiencies.
I'm assuming that the second safety flange is designed to help keep the bead of the tire against the edge of the wheel. I don't run the original Brand Lotus wheels now but rather 3-piece Compomotive wheels. I don't remember what they looked like without tires on them but looking at the lips, there does not appear to be such a second safety flange and I have no problems with my tires.
Correction: I had a wheel off this afternoon and noticed on the barrel section (inner part of the wheel) there seemed to be a safety flange so I'm guessing they are on both sides of the wheel. And since it's obvious I don't mind being wrong, I'm going to suggest that those safety flanges or ridges came after, maybe long after, our Europas. I don't remember seeing them back then.