The subject of these panels has come up previously. On most XKs, they seem to have quickly rusted away, as they are made of untreated thin sheet metal. They are still intact on my XK120, which has spent most of its years in dry Arizona. In the above link, there are chassis numbers and photos of what appear to be original XK120 and XK140 panels.
Here is the underside of (XK150), Chassis No. S838240, showing the panels circled in green. The uncovered openings between the panels and door bottoms allow access to the bottom side of the lower hinge boxes.
