CENCO circa 1950. About 10 inches tall, and occupying about 6 1/2 inches square on the table, this CENCO Scientific plaster case of the heart is in nice condition (a few minor breaks, and chips but otherwise quite nice). These didactic models were used in classrooms to give students a better three dimensional feeling for how the heart worked. Two dimensional textbook images just don't do as good a job.
This example has a removable front allowing the inner chambers to be viewed. Each structure is numbered, and presumably there was a manual (not present here) explaining each part of the heart and it's function. Very Good. 
Dating these is a little tough since CENCO has been around for years. Their website notes the following: "Started in 1889, then called Central Scientific Company, it has designed, manufactured and distributed instruments for analyzing Moisture, Particles, Surface Tension, Viscosity and many more material attributes.
Early on, it was a leader in educational apparatuses used for teaching science. It became known by its trade mark CENCO which stood for CENtral COmpany. The CENCO mark is still used on apparatuses that are made to demonstrate physics principles.
Among other highlights of its history was establishing the concept of not only manufacturing propriatary instruments but also supplying products of wide ranging producers. This permitted CENCO to offer families of devices to solve specific measuring problems. It could be said that this parent of CSC Scientific Company was the Grand Daddy of the Laboratory Supply Industry."
In our experience the later models were plastic, and we'd put this one around the 1950s. A little research could do a better job of dating it I'm sure, but that's a conservative guess on our part. No matter when it was made, the plaster models are much more desirable, and occupy a spot in the hearts of most teachers who used them!
See the photographs for more details on condition.