This is a small brace to support the SMA to BNC adapter from Signalstuff.
Adapter can be found here: https://signalstuff.com/product/smam-to-bnc-adapter/
This is designed for the (tr)uSDX by DL2MAN and PE1NNZ. It simply slips over the SMA connector to brace against the side wall.
The author marked this model as their own original creation.