Prepare for gluing
Several counter-forms are needed to successfully hold the head in place as the glue dries. When hot animal glue is applied to the mortise the water in the glue swells the inside of the thin pegbox walls, causing them to flare outwards. The visible edges of the joint no longer meet and all hopes of an invisible join are dashed. Very close-fitting counter forms are needed hold the pegbox in place. Before applying glue I cast a pair of counter-forms from dental impression compound backed with wood and lined with masking tape.
Another detrimental effect of the fresh glue is to lubricate the joint so that the tenon slides in past the location it was fitted to. This would result in spreading the peg box out past its original width. A small wooden block has been glued to the back of the tenon to act as a stop.