Thanks to everybody for replies!!!
Things are much clearer now. I was not even aware that the piece in the drawtube unscrews :-)
Now the only thing I am not sure about is the t-ring size. The thread on my t-ring has 42mm diameter. I have seen some which are sold as M48, for example
this one. I am just not sure if this is what I need as 48mm is still few mm short of the mentioned 2" (51mm).
Can someone please put a ruler across the middle of the thread on the reducer or the field flattener and let me know a rough measurement. Or alternatively measure the diameter of the thread on the t-ring that fits the reducer/field flattener.
Thanks again for great help
Luka
edit:
Don't worry about my post above, M48 seems to be the right size.
I have also found M42-M48 adapter
here. Half price compared to a new M48 t-ring.
Will get it together with the Orion field flattener.
Thanks again