The plastic Swish system connector brackets were available in two sizes:
The 15cm sizes, pictured below were still available until 2017, after Sunflex who bought the Swish brand discontinued them.
Personally I thought it was a good system, though for light curtains only. The video below shows how they worked in conjunction with the lever lock brackets which are still available
As of 2019 it looks like the metal Swish extension brackets have also been discontinued. Certainly major wholesalers have run out of stock. Being critical, since Sunflex took over the Swish brand, the availability of spare parts has been inconsistent.
The metal extension brackets used a system connector bracket and a wall box. This was a surprisingly sturdy system, though my recommendation would be for up to medium weight curtains.
They were available in three sizes:
The photo of the 10cm variant includes the modern system connector bracket and wall box
and the video below shows how they worked, both with the new and old system connector brackets
The metal extension brackets were