For keeps, I would definitely take the neck off & thoroughly inspect the block. Make a gizmo to apply load widening the mortise while you can still see it, and see what happens under load (not excessive). Maybe you can repair it in-situ, otherwise it's major surgery. I fear it may have to come to that. The crack between the rib and the back is clear, but it seems the seam between the top & the rib also shows a crack. The rib is probably still properly attached to the outer part of the block, but that would mean that the connection between the block & the top & back is at least partly lost. For now I can't completely see how the repair would best go, but my initial approach would be to take the neck off. FB first indeed.
Yes, before all that, I would also like to hear what I had bought (although this sure looks like an interesting box; congratulations on your find!). Be prepared that the heel joint may break, or that the pegbox catapults off. Prior to tensioning, use masking tape to make a bumper at the end of the FB (from cloth or neoprene or so), to prevent damaging the top. Wrap a towel around the pegbox. Maybe also some cloth + tape around the end of the TP. Consider first tensioning only 2 or 3 strings. SP OK?
The pegbox (hence the entire neck) can be saved, if you so wish. The choice is yours, depending on the cost of another neck & your view on the value of the instrument after the repair. Of course a pristine neck will enhance the value, but then the last chunk of heel needs also to be removed from the mortise (if keep same, I would probably leave that).
The crack in the pegbox cheek just below the scroll is insignificant; it gets little load. I do not know how to fill such a narrow crack with HHG, it will require opening-up & shimming, or a graft. Those would be the conventional approaches. Alternatively you can consider flooding w/ CA, that is easiest but it would
not be my first choice. Rather than CA, I would prefer using a 1-component PU (low foaming). Put some in a syringe, and make a seal from a piece of rubber on the tip of the syringe, so that you can inject it into the crack (by pushing onto the outside). Start somewhere in the middle of the crack, and inject (bending it open with your other hand, as much as it goes) till you see it coming out on the inside, and work the entire crack likewise. If the viscosity is too high, use a very little bit of acetone to dilute, and mask the area outside the seal & inside the pegbox. When properly flooded, wipe & clamp lightly. (It may also work to massage the PU in with your finger, but you may not get deep enough due to the low pressure achieved that way. Use gloves. Messy job.)
The fracture at the pegbox heel gets full string load, hence
is significant. Do
not use a foaming PU here, and certainly
no quick epoxy. Looking at the cracks in the back of the pegbox, breaking it to gain access does not seem a good idea. I'd probably open it up & and shim it with HHG, and add reinforcements on the inside (like in my last pic in
Putting the head back on , or triangular reinforcements if you prefer). I don't advise using dowels or screws, though you do see those used in situations like this. You could consider using a
slow epoxy without opening-up, but the problem is getting it in. In either case, the contaminating old glue has to be removed first.
Just my views, hopefully there are others.
