Note: Contested against a published dressing: The pattern is named for its body material and we have the wrong one. The source body is Arizona Mega Simi Seal in the "California 420" colour; we store "micro chenille or dubbing, olive/green mottled". Source: Fly Fish Food — "California 420 Leech" (stillwater tutorial, 15 December 2020) — https://www.flyfishfood.com/blogs/stillwater-tutorials/california-420-leech
Reference Recipe
Hook jig nymph, 10-14
Thread olive 8/0
Materials
Bead Gold slotted tungsten (1, sized to hook)
Tail Olive ostrich herl (marabou + few strands pearl flash)
Body Olive/green mottled material (micro chenille or dubbing)
Collar Olive UV material (ice dub or marabou)
How to tie it
Slide a slotted tungsten bead onto the jig hook and start olive thread behind it.
Tie in a slim olive marabou tail about one shank long, with a few strands of pearl flash.
Tie in the body chenille (or spin a dubbing loop) at the tail.
Wrap a lightly tapered olive body forward to just behind the bead.
Add a short olive marabou or UV ice-dub collar and sweep it back for movement.