The Tiger E was originally designed with a HL 210 engine, mounted on the center line of the hull, in the rear.
For access to the bottom of the engine, two round holes were cut in the belly plate. This diagram shows the hull from below.
Steel cover plates were bolted into the holes, flush with the belly plate. They had 8 bolts each.
Looking down at a hole from above, we can see that two curved metal strips are welded inside the hull bottom plate. It is easier to make threaded holes in soft steel plates like this than in the hardened steel of the belly plate. (This is Bovington's tank.)
The openings were 250mm in diameter. The innermost struts welded to the floor are just clear of the threaded strips. The inside strips are trimmed to avoid the ventilation ducts.
When the HL 230 engine was introduced, the left-hand access hatch was replaced by a larger rectangular hatch.
[1] Survey of Tiger 250122, at Bovington museum, by David Byrden