Because the bolt isn't keyed or a carriage bolt it simply spins round and round as you try to tighten it. Huge waste of money.

The GSD Gen 2 most definitely has a 40.1mm lower collar diameter. Very weird error to have written here. Even worse, there is no indication on the packaging or the collars themselves of which version or of the diameters of the product.

Don't buy this if you need to adjust your seat up and down often, but if you don't then this is a great way to keep your saddle and post from being stolen.

The Pinhead locking system is very clever, but the seatpost locks have a major design flaw. The bolt head has a knurled surface to provide friction with the sleeve, but it is insufficient to allow more than a few adjustments. Soon, the keyed nut cannot be tightened sufficiently, because the bolt starts turning. I complained, and Pinhead sent me new bolts and nuts. But the knurled surface was gone - replaced with an ugly lockwasher. Since the lockwasher is larger than the recess in the sleeve, the bolt head is accessible to pliers, compromising the security of the lock. Or it would, if the lock could be tightened in the first place. The lockwasher just rotates against the sleeve, or the bolt head against the lockwasher when trying to tighten. IMO the bolt should be a carriage bolt (square shape under the head), and the sleeve should have a matching square hole. If they should redesign it this way, I would recommend it to others, As-is, it is almost useless, and a waste of $80.