...but sometimes serendipity strikes!
I found some sill cover plates for a good price =>
They are made from some sort of aluminium alloy - meant to be shiny - no rubber or plastic has ever been applied to the upper surface (evidence of bitumen goo on the underside).
The seller said they came off a PV444 L series so that's between January 1957 and August 1958 - the last of the PV444s. Whether these sill plates were fitted on earlier versions or not, I do not know - whether these were factory fitted on the PV444 L series must also be questioned a little bit as the seller could only say where he had got them. I think the chances are good however...
...these PV444s are proving to have interesting bits on them! First the nicer sun visors - perhaps a nicer grill (?) - and now a reasonably priced solution to the sill cover problem.
Happy days.