These are top quality tungsten beads at a great price with small slots to increase weight.
We label the beads the size that they actually are while most dealers just round them up to the half mm size. What that means is our 2.3mm bead is another seller's 2.5mm and our 2.8mm is another seller's 3mm bead etc. Slotted beads are great for jig hooks but I also use them for straight shanked hooks as well. By inserting lead wire into the slot you can offset the bead to increase hook gap and the hold of the hook while also inverting the fly in the water. The colored beads allow you to add an extra flare or a hotspot to your flies. The 3.8mm beads fit into the FIPS Mouche measuring device so you'll know you're legal if you fish in FIPS governed competitions.
See the hook and bead chart in the product photos for information on matching hook size and bead mass