Ah, a PO Box.
I think it must at least predate Fender, as Fender only needed original purchase receipt and didn't
require mailing in registration card.
And I think I'm remembering why New Berlin rang a bell: I think it might have been where US Musical Instrument Corporation(?) was registered.
Think they bought out Gruhn so would be late '80's but don't recall for sure and can't recall where that history's found, but pretty sure it was some time in '80's, and don't believe is (was?) the same entity as US Music Corp.
Hope that helps you narrow down the search, at least.
****Ah, I see sailingshoes nailed it while I was composing.