I think it's classified as "Structural Clay Load-Bearing Wall Tile", and further, "Grade LB-"Suitable for general use in masonry where not exposed to frost action, or for use in exposed masonry where protected with a facing of 3 in. (76.2 mm) or more of stone, brick, terra cotta, or other masonry."

From http://www.astm.org/Standards/C34.htm

Casey

Here is a link that might be useful: clickable

Thanks Casey

That seems correct.

Just purchased a 1927 bungalow. Kitchen floors are sloping. Can we somehow even up the floor before tiling?

Since ASTM is unlikely to have had a standard in 1930, it is unlikely to be what is in any standard now.

Many masonry houses built in the 1930s had non-weight bearing hollow block inside of a double thick brick wall.

The block had an air gap from the brick and horizontal air channels and was a vitreous product (though often not glazed) designed to prevent excessive wicking of water.

The inside surface was molded with about 3/8 x 3/8 grooves to allow plaster to be directly applied to the block.

The exterior walls of these masonry houses have no studs of any type in these walls.
Three coat plaster was applied directly on to the block.

It is a PITA to cut openings for electrical boxes, but the air channels do provide a convenient place to run new horizontal cables without having to trench the plaster.

Can we somehow even up the floor before tiling?

Yes. Use leveling compound.

Brick is obviously correct as always. I could not locate any 1930's web pages ;-).
I grew up in a solid stone house built in the 30's . It had 24" thick walls. They were lined with these same kind of tile, and plastered with cement (not interior plaster, this stuff was sand + rosendale cement!) I still have the old B&W photos showing the hollow tiles during construction.
I worked on the restoration of a house in 2000-2003 that was built (circa 1910) entirely of these tiles in a load-bearing form. They had the grooved surface for plaster as Brick describes. And they were stucco'ed on the outside. (the architect was Waddy Wood)
This stuff was extremely popular from 1900 onward. It differs from concrete block/cinder block in that it is fired at high temperatures and is damp-proof as a result. Therefore, the house is more comfortable and less prone to damp/rising damp than a concrete block type.
The structural/load bearing block, as in the Waddy Wood house, is unbelievably hard, even a large-size hammer drill has to work to penetrate.
Casey

Thanks brickeye for this correction. We can only see this tile at the back of a cleanout for fireplace ashes in the basement and have experienced its hardness when we tried to install a wall mount for a tv on an outside wall. Actually we will see it soon when we demolish a mud shower as one of its walls is an outside one. I think we should use mud again for this shower construction on the walls and the pan but finding someone to do it is another thing.

I am also looking at purchasing a 1930 bungalow type house in Mississippi that appears to be brick veneer with plaster walls on the inside. The floors are not in good shape and may have termite damage. So one though would be to rip up the floor and pour between the walls with a concrete (slab on grade) also put in all new plumbing and electrical as this will need to be placed also.
Would like to add an addition master bedroom/ bath and kitchen and prefer to have in-floor heating through out the house.
Would this be feasible?