Ok, as far as I'm aware, the "Slab" P Basses were exclusive to the UK yet, whilst browsing the book "American Music" by Annie Leibovitz, I stumbled upon an interesting image.
On page 82, there is an image of a blues player named Johnnie Billington. Taken in 2001, this image shows him with some of his students at his blues school. The young bass player is clearly playing a slab-bodied p bass with a gold anodised pickguard.
I've attached a copy found on Google which is a little hazy but in the book, you can clearly see the slab body! Of course, it could be retrofit or even one of our 66's that may have found its way over the pond but I just thought it looked interesting.