I bought my A3 31 second hand from a member of this forum....It came with a geniune Hammer / Felder 120mm to 100mm reducer.
I use a dedicated extractor for the machine and it only has a 100mm pipe connection. Dust extraction is very reasonable,...Not 100%, but I wouldn't have expected it to be considering the reduction in the adapter & hose size.
Here's a link to the one I have......Alternative's can be found cheaper online, but this is the genuine article.
Thank you. Found a reducing sleeve in my local plumber's merchants that has done the trick nicely. Shame the people at Felder in MK didn't have the foresight to try and sell me their reducer - one collects the machine fully wrapped, so determining the outlet port size was not possible.