What is the object on the tail of this F-16 (next to the serial number)?
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Source: primeportal.net; cropped
That's the cooling-air outlet that is aft of the cooling inlet. In between is a power amplifier for the radar-warning system.
Source: amazon.co.uk (one of the product images); cropped
118: Cooling-air ram air intake
124: Radar-warning system power amplifier