I'm not sure all M23 came with factory choke tubes, just Googling around now I have found some evidence that some had fixed chokes.
I think the first thing you need to do is determine whether or not yours came set up for tubes from the factory, if so then you can easily source replacements.
However, your guns tubes, being extended tubes, may have been aftermarket tubes. If so you will need to determine what thread was used for the aftermarket tubes and go from there to find out what flush mount tubes might work.
