Unfortunately not, and fuck knows where it is. But the base is recessed about an inch (you can se from the line of screws along the front, not exactly award winning joinery) and I used these castors.
I was thinking to route a groove and let in some 5.5mm ply for the base, attach the castors underneath, so a similar principle. The advantage of the nylon rollers is that they go in any direction.
Do you have a photo from underneath?