Broken Sturmey-Archer dual cable 5-speed shifter
It looks like these are mounted in the same way as downtube shifters. If that is so, many downtube shifter band clamps are available aftermarket for the frame, stem or handlebar. If you take the shifters off your clamp you should be able to compare photos of this type of solution
See examples:
https://www.bricklanebikes.co.uk/via-downtube-shifter-mount
https://southerndistributors.co.uk/product/quill-stem-shifter-mount/