Dave Blundell sez "Vernon (deluded) sent me much of this info."
As a rule of thumb, 90 PR4s have external PA sensors, and 91 ECUs have internal PA sensors.  You can tell (to some degree) by looking at the "code" of the ECU:
 |  Label  |  Description  |  
|  A00  |   90 5 Speed (-33 program)  |  
|  A01  |   90 5 Speed (?? program)  |  
|  A02  |   90 5 Speed (-54 program)  |  
|  A51  |   90 Automatic (?? program)  |  
|  A52  |   90 Automatic (-54 program)  |  
|  A10  |   91 5 Speed (-92 program)  |  
|  A12  |   91 5 speed (-92 program)  |  
|  A60  |   91 auto (-92 program)  |  
|  A62  |   91 auto (-92 program)  |  
 
Ok, with that out of the way...  It is easy to select whether to use a internal or external PA sensor.  See the following table:
 |  Part  |  External PA  |  Internal PA  |  
|  PA Sensor  |   missing   |   installed   |  
|  C103   |   missing   |   present(103Z)   |  
|  R92   |   jumper   |  10Kohm   |  
|  C106  |   missing   |   present(103Z)   |  
 
*edit*
It seems that the Internal PA ECU is missing C106, and the External PA ECU has C106 present.
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