It could very well be a press fit that cannot be removed without heating the flange. Upthread, Steven mentioned that someone might have used a blowlamp (blowtorch in US) to remove the original mounting flange. Some more speculation:
I took a machinist's class in college (physics department shop) and we made a practice dingus using a press fit. An aluminum cylinder was fit into an aluminum disc similar to your flange. We machined the hole in the disc to have close to zero clearance, that is the hole was the same size as the cylinder. With zero clearance, you can't push the cylinder into the hole. We then cooled the cylinder and heated the disc - I forget how hot, but not red hot. Aluminum has a relatively high coefficient of thermal expansion for a metal, so this makes the hole just big enough for the cylinder. We then quickly set the two pieces together and slammed them home with a hand press. You only have a few seconds to do this, because once they are in contact they come into thermal equilibrium pretty quickly, and then they're stuck for good. Apparently press fits are pretty stressful even for experienced machinists.
https://en.wikipedia.org/wiki/Interference_fit
Because your flange is aluminum and the lens barrel is likely brass, and aluminum has a slightly higher coefficient of thermal expansion than brass, you
might be able to heat the flange enough to expand it and press it off the lens. I would be super careful with this because I think getting the cemented glass elements hot would be bad. (It's the cement, black paint, etc that would worry me; the glass itself can probably take a pretty high temperature).
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