This is a shell and custom stem for a B&M Cyclestar mirror. The ball end of the stem inserts into the mirror socket. The straight end inserts into the top hole/boss in the shell. The stem has been left long so you can cut to the length to fit the mirror in the shell where you like it. The hole in that top boss has been left a bit small so you can drill to get a tight fit for the stem. The shell is attached to the velomobile with one or two 5mm bolts and nuts through the base.
This mirror shell looks a bit funky straight off the printer. I'd suggest sanding and painting them.
I've printed these in PLA. If they crack, I'll switch to ABS or nylon. The stem should be printed horizontally in nylon for strength.
One last thing... My printer may not be calibrated perfectly so you may have to tweak the dimensions a bit.