Relief Valve on Discharge Line by: Bill - Publisher
Dave, a two cylinder compressor is often a two stage compressor.
The air gets preliminary compression in the first cylinder, and then that already compressed air gets further compressed in the second cylinder as it sends air into the receiver.
Since PRV's are used in many places to ensure that over-pressurization and catastrophic failure of a pressure vessel can't occur, the engineer that designed your compressor decided that the cross over line was where one was needed in that compressor's design.