Frozen screw...

[M1 Thompson]

If the forend is indeed scrap why not just remove the whole of the volley sight plate & screw assembly from the wood? You can get at the metalwork then. The volley plate screw is cheese headed, incidentally. The threads are likely rusted into the volley plate though there may well also be some adherence of the corroded shank of the screw in the wood.

Even if the forend is damaged further by the removal of the metal, it still has uses. I use such scrap pieces of woodwork as 'donors' for patches needed on otherwise serviceable woodwork. For example, the raised & rather thin piece of wood that often breaks off, located at the right rear of the forend between the charger guide recess & the very back end, can be carefully salvaged & grafted onto a needy piece of wood (glued & dowelled/brass wired)........pieces can be cut out of the area of the trigger guard recess & grafted on to another piece so as to make it usable again......& so on. Waste not want not!

That's exactly my plan, Roger.