What do primordia/pins look like? I don't know...

The growing of the mushroom from the cake starts as a tiny accumulation of mycelium, called hyphal knot, which then develops to primordium, and this further develps to pinheads.

In the picture of a casing surface underneath, the primordia are the small white bumps scattered across the casing surface, and the pinheads can be seen bottom left and bottom middle. They already have small brown caps.