Concrete blocks, unknown type [CB99] - jcgomezz/Building_taxonomy GitHub Wiki
It is clear that the masonry unit is concrete block, but the type of block is unknown. The blocks may be hidden, or it may not be possible to determine solid from hollow blockwork, or information about it is unavailable.
Various types of concrete blocks, Peru (S. Brzev)
Concrete block construction, Cuba (S. Brzev)