Just picked this up. It was bought from Thomson and Doxey in Southport, north of England, about twenty years back and that owner couldn't find a use for it.
... it's about 18" long, 8" wide, and 7" high, weighs no less than 50lbs, maybe as much as 60. Mounts on four 3/4" holes on 4" by 4" centres, and has " ORD-D 76899 " cast onto the bottom ? face.
I'd guess from the mounting lugs on the rear face that they slot into either very thick plate or armour.
Only thing I could come up with would be for some sort of big towed artillery piece, but it could be for a plain trailer, so I'm posting this here.
Any ideas?