Mando,
The're just a good looking flanged bolt, very factory looking, i like the look better than using a washer.
Lots of sizes so you can get just the ideal length.
I use the flanged nuts as well, they have a slight serration in the base that locks them up against vibrating loose.
i get them from here by mail;
https://www.bolt.com.au/bolts-durlok...1011_1042.html
Dave