Thank you for your help. Meanwhile, with the help of Andrew from SY Ronpische, I managed to get it off. Maybe Bill Rouse can include this in his fantastic "Bible".
After opening the winch you can see the 5 screws holding down the tower.
Those were rather seized and difficult to open which made me think they where fixed with nuts (like the 2 16th winches on the mizzen). They are just screwed in the base of the winch, with a very large screwdriver and a lot of force, I managed to get them out and get the tower of. If you have to go further, you have first to get off the electrical motor (or rather the reducer of the motor). To do this you can reach 2 bolts from the small opening on the side of the mizzen base
and 2 more through the base of the winch, behind the white silicon plugs (one is already off on the picture).
Now the motor comes off at the panel in the aft head near the mirror an you have access to the nuts holding down the base off the winch.
SM2K 426 RedLion