The block laid bare - left side
The very earliest engines had a different oil filter head that mounted vertically rather than on a slant as is the case here. Apparently the filters for those earlier ones are a bit harder to find and quite expensive.

Contrary to popular belief the block is
not the same as on a Triumph TR2, 3, or 4, although there are indeed some similarities. See my page regarding the connection for more detail.
On to the engine serial number issue