Can anyone point me in the right direction for filling the selector cut-out on my USGI Birch stock? I was just going to use wood putty, or fit a scrap piece of wood and glue it in, then sand it flush.

What's the best technique from you pro's on here?

faux selector

I would open the notch out to a square and splice in a piece of wood. Then knock it down with a small block plane and sand/scraper it flush. If done right you woould be hard press to know it was there.

Not doing a faux selector. Thanks guys.

I use bedding compound (your choice of what brand). Tape a piece of smooth plastic to the inside and outside of the selector cut out.
With the stock being level, pour the bedding compound in the cut out and let it cure. Remove the plastic pieces and file/sand the bedding to shape.
While the faux selector or wood plug both look great, the first place a wood M14 stock breaks (splits) is from the selector cutout to the stock liner screw hole.
By using bedding compound, you have greatly strengthened the stock far more than if you glued in a wooden plug.

I had the same train of thought - no joke!!! Thanks for the advice!

Be sure to include a couple of pieces of rebar (nails or screws) to re-enforce the filler. There is actually a lot of stress in this area and it's common for the filler to crack if not supported. This is especially true in glass bedded stocks.