It is just a frame handle construction done with wood/antler instead of metal and wood/ivory.
The following instructions start as any three piece frame handle done in wood. This is an excellent method of making a tight fitting handle for a take down or an odd or curved shape tang assembly. Directions are specific to the OP koa/elk idea, but would work in any material or combination of materials.
Start with a 1.25"-1.5" thick block of koa just a little larger than the desired handle assembly. ( the extra thickness is to allow for the two saw cuts. Use a thickness that is sufficient to get the desired final handle bock) Sand the edges smooth. At this point I place random diagonal lines and "X" marks along the sides and ends that will match up only when properly assembled later on.
Cut it in lengthwise into three scales. The middle scale should be just a tab thicker than the tang. Sand it down to about .020" thicker than the tang. You now have two outer scales and an inner frame scale. Sand as needed with 120 grit paper to assure the mating surfaces are perfectly flat. Mark them 1,2,3, and designate front and back as well as inside and outside so they can be re-assembled with correct grain lines later on. The "X" marks made earlier will greatly help alignment. Re-assembled properly, the joints will be invisible on koa. I recommend using Tightbond II for dark wood.
Now - for the start of the wooden frame handle:
(In a regular handle assembly, you would just glue the two outer scales on the frame pieces. You can do that here and shape the inlay sections to fit the elk slabs if you have a mill or good woodworking skills. If you are new to all this, the method given later on may be better for you.)
Lay the tang on the frame scale and draw the outline. Cut the scale in two pieces along those lines. (Save the inner piece, as it can be used to clean out glue during assembly)
Now you are ready to attach the outer scales.
Start the sandwich by gluing one scale on the frame pieces. Make sure you have aligned the marks on the sides to match the grain. Clamp with firm clamping all along the seams. I like the 3" and 4" black clamps from HF that sell for less than a dollar each.
When dry, remove the clamps, and test fit the tang into the channel to check the fit. File/cut out any places that are too tight and remove any dried glue squeeze-out. The tang hopefully is a nice fit now.
Glue the other scale on aligning all lines to make the original block a good reassembly. Clamp well. Run the tang in and out. Wipe the glue off the tang after each insertion, and repeat until it comes out clean of glue. Set assembly aside to fully cure.
When cured, check the tang fit and file or scrape any glue squeeze-out or tight places to get a good fit.
At this point the frame handle is ready for inlay and shaping.
For a simpler inlay assembly this will work well:
Trace the tang on a piece of stiff paper. Brown bag or craft paper is good.
Place one of the outer scales over the drawing and draw the outline to show where the tang will sit inside the final handle block.
Spot glue the two frame pieces to the paper using a couple tiny drops of the wood glue ( they only need to stay in place for the next steps). Align them so they fit the outer scales marks, not the tang mark ( which should be close, but aren't as critical as the outside).
Trace another outline of the outer scale on a sheet of printer paper, and mark where the inlay of elk will go. The wood in front and behind that are the bolsters. Set this paper aside.
Now is when you will make the front and rear bolsters.
Glue the two scales together with a little glue only in the middle ( where the elk will be). Use two small drops of CA for this. The end parts that will become the front and rear bolsters should not have glue between them.
Cut the front and rear bolsters off the glued up scales. Set them aside on the paper with the layout drawn on it to keep the front and rear bolsters straight so the final grain will line up later on. (If the bolster pieces got accidentally glued together, they can easily be split apart with a sharp knife blade once cut off the scales.)
Take the top front block of bolster koa and glue it on the frame strips where the front bolster will be ( Remember the original 1-2-3 scales alignment and make sure the pieces are aligned with the grain marks). Use the Tightbond wood glue. Use medium tension clamps (I like the 3" and 4" ones from HF that sell for less tan a buck each). Let dry overnight.
Use a safe edge file and clean up the transition where the inlay piece will go. You want the joint at 90° for a simpler assembly. Rounding the joint will greatly increase the fitting difficulty of the inlay, but can be done if you wish ( If you decide to round the inlay, you were warned).
Cut both inlay slabs of elk and shape the ends. Again, 90° square ends will be far easier to do. Set one aside on your block drawing with the bolster pieces.
Place the elk slab on the frame. It should fit as closely as possible against the front bolster. Glue on with Tightbond, using light to medium tension clamps. Once fully cured, file the elk end to remove any glue squeeze out, and add the top rear bolster block with Tightbond glue.
Remove the paper backing and sand the frame lightly to remove any glue bumps.
Clean out the tang channel of any glue drips, and check the tang fit.
Glue on the second side front bolster/elk/rear bolster as previously done.
At this point you have a rather ugly assembly of koa and elk. Trim it up on the grinder to even out the sides and top. Now you have your rough inlay handle assembly. Check the tang fit and scrape/file/broach out any glue that is in the channel.
Shape as any single block of wood to make your final handle.
