Doing a neck job on my Strat and noticed there is a round drilling about 5/8" on the neck pocket (cavity). Does anyone know what the purpose of this is? Is there an acoustic purpose to it?
I was guessing it is something they do to hold the body while finishing the guitar. Really don't know. Was thinking to fill it up with putty or something.
Does anyone know? Thanks

It's a CNC indexing hole. It's drilled into the body blank and the CNC machine uses it as a reference point while cutting/shaping the body. Some models have that hole plus other holes under the pickguard. Those are indexing holes too, though I don't exactly know why they use different hole patterns.

Some reissue models don't have any large unfilled indexing holes (Custom Shop Time Machine models have small holes filled with dowels). Those are cut using the old pre-CNC methods.

There's no reason to fill the holes. And even if there was, it would probably be better to to cut wood plugs from the same type of wood as the body rather than to use putty/filler.