The Therm Products C2400-0321-A heater assembly can be found in Cal Spa hot tub systems (but not all and not limited to).
The ThermCore C2400-0321-A flo-thru heater assembly has the following features and specifications.
Additional ThermCore Part Numbers: C24000321-A, C24000321A
Manifold Length: 15.5"
Manifold Diameter: 2"
Heating Element Included: Yes
Kilowatt Rating: 4.0KW
Voltage Rating: 240V
Mounting Studs: Yes
Number of Mounting Studs: 3
Clip for Sensor: Yes
Temperature Sensor Included: No
Hi-Limit Sensor Included: No