This is the optional bottom bracket for the model here
https://makerworld.com/en/models/183781

The PTFE arm can be attached either without this piece, with zip ties, or by using this piece and M4 bolts.
The nuts are embedded within the bottom bracket.

The M4 bolts should be at least 25 mm long, but preferably 30 mm long. 

I recommend using PLA or PETG.

I used a thin sheet of rubber between the A1 top bar and this bracket to add some compression and keep the arm in place