This is an M20 x 1.5 nut with a spherical narrowing at the bottom. It is a replacement part for the Scosche MagicMount Pro2 Dash/Window. The original nut broke. It gets hot where I live and I don't think the plastic they used for the original nut was up to the challenge of heat AND the stress applied to it. This nut ends up being an upgrade anyway as it's slightly larger in diameter and has better star features for grip.
I recommend printing this out of ABS or ASA, but PETG can be a permissible alternative as well. Avoid PLA.
The author marked this model as their own original creation.