bdial, thanks, this is a very helpful hint as I didn't know if I had to get the part with the screws off

Bob-D659, thanks for the comments. You are certainly right about the rear panel. It seems as it has been fixed with glue. I am loosing any hope to get this lens off as I tried all your tips.

I wonder if I could saw the cylinder off without harming the lens... will probably try that.

Thanks again