Aubergine-
TA racks came in 3 versions: without a light support wire, with an unthreaded L shaped wire for a clamp and with the threaded boss that you have at the end of the L shaped wire. I had an identical one and it held a Soubitez headlight securely. The Soubitezs came with a serrated washer crimped to the light housing mounting bracket and mounted right up securely to the TA boss. They aren't terrific for light pattern even with an LED replacement for the OEM halogen bulb. Some specifics
here, if you need them.
I added a photo below showing the Soubitez mounting tab.