[G6Tanuki]

Looking at the [barely readable] S27 schematic I have here, it appears that the headphone-socket wiring also has association with the grid-circuit of one of the push-pull 6V6s, as well as a contact that would appear [reading through the scanning-artifacts] to open-circuit one of the O/P Tx secondary-windings-to-rear-binding-posts.

Plug in [high impedance] headphones and get them connected to the grid-drive of one 6V6 and at the same time disconnect the speaker?

I really need a more-legible schematic (and I use that particular word precisely because it's what Hallicrafters would have called it...)