While exploring the electronics cavity of my recently acquired 1979 Series 1 SSB I discovered that the lower part of the body is hollow all the way to the lower horn where the pickup selector switch is located. There is also metal tape/foil all the way to the PU selector switch which looks original and undisturbed. See photo...
Is this standard design or something unique to older basses or just mine?

Hollow Core Construction.  Series I and II basses and guitars share this construction. Many Signature Deluxes do as well.

Fascinating! Thank you for the enlightenment, dear Mica!
The world of Alembic is a inexhaustible source of amazement.

I was going to tell you a squirrel must have got in there and nested but Mica's explanation is probably more accurate. Congrats again on the bass. You are going to love it.