Yeah, the K4 standard calipers don't have locating dowels (i.e. collars). They just rely on the bolt. I always wondered about that too.
If you're good with hand tools, you could probably file those collars off. Keep the file straight and get it close. When you're almost there, get some 120 grit wet and dry, place it on a flat surface and rub spacers back and forth to get it an even true surface finish.
BillV said the bolt is 10mm, so make sure the internal hole is also 10mm.
Looks like you are using an aftermarket bolt. Make sure you still have enough thread going into the forks but also not bottoming out. Like craigfer63 said, you may have to pull out the vernier calipers and measure things up.
(NB. Brakes are important. I believe they should be left OEM. But if you do go aftermarket, follow
the manufacturers' instructions.)