Not the Pope, I hope.

Bill Lawrence

Acraglas dyed black can be almost invisible if the mend line is clean with no large crack.  The black looks like dark grain lines, and if the break follows the grain, it is even harder to tell it isn’t natural.  Brown dyed Acraglas on the other hand usually looks out of place.

The seller indicated that the screws would not turn and left them alone.  Wanting to see the extent of the rust and not being able to remove the upper tang screw either, I thought maybe there was a through bolt.  
When I finally got the lower buttplate screw to turn, the large piece broke off.  Looking at the break, it appears it was only attached in the forward circled section.  Massive amounts of rust had destroyed the screw which probably caused it to swell and caused the crack.  The upper buttplate screw had to have the head drilled out.

The tang cleaned up OK.  Luckily none of the other internal parts showed any evidence of rust.

You probably will need to clean the cracked area with acetone to remove oil from the wood before gluing.  If the pieces fit together tightly, I’ve had good success with thin superglue for areas that don’t have a lot of stress. The thick superglue doesn’t soak into the pores of the wood like the thin variety does.

I have made repairs such as yours. If there isn't a gap to be filled then I have had good success using Tightbond wood glue. If there is a gap that needs to be addressed then either Brownell's Acragalss or a Acragel. I would use a Qtip with paste wax and coat the area around the crack that doesn't need to be glued to help with cleanup. If using Acraglass or Acuragel I wait until it sets up but not 100% hard and trim the excess with a sharp razor blade. Once set up hard it is like a rock.
A. J. Palik

To make that repair as aesthetic as I could, I'd do it with dyed acraglass.  As someone else said, clean with acetone to remove oils.  Pre-fit the piece, remove minor amounts of wood from the interior as needed to close up the crack as well as you can, if splinters or loose pieces are holding it open.  

Set up your clamping system ahead of time and make sure it works BEFORE you get epoxy on the pieces.  I use a combination of C clamps, F Clamps, rubber hose stretched and wrapped around the stock, and spring clamps.  Whatever works.  You can put masking tape an eighth inch away from the edges to keep epoxy from smearing onto the wood.

When it comes to the actual acraglass repair, I would mix 2 or 3 different batches of brown/red/black dyed epoxy.  Try to have all 3 darker than the wood, and more toward black than red/brown.  Then swirl the 3 together slightly on a glass plate, and use a knife blade to scrape up a dab at a time, smear thinly on the wood on both sides.  Especially along the edges, mix and match the different colors so it varies continuously along the length of the crack.  Get full coverage well adhered to the entire wood surface on both pieces before putting the broken piece back in place.  

Clamp, and use a blade to scrape off what oozes out of the crack.  Wait 10 minutes, and come back and check it again.  If any shrinks back into the crack, refill flush with the surface.  Sometimes the epoxy runs and you have to relevel it several times before it finally sets.

When it is just about fully set, but still slightly soft, I use a sharp razor blade to cut it all off flush with the surface.  Let dry fully for several days and then lightly sand smooth and apply a bit of finish to touch up if needed.