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The HP 8657D Signal Generator with Option H12, H35 includes the π/4 DQPSK I/Q Modulator.  The original cost of this option in 1993 was $23,625.

This adds π/4 DQPSK digital modulation capability to conventional AM, FM and pulse modulation. It has the capability to test mobiles for the North American Digital Cellular (NADC) and Japanese Digital Cellular (JDC) radio systems.

HP 8657D with Option H12, H35 DQPSK I/Q Modulator Unit
DQPSK I/Q Modulator front view
DQPSK I/Q Modulator rear view
DQPSK I/Q Modulator top view with cover removed
January 2016

The Signal Generator and DQPSK Modulator were separated and cleaned up.

A new power supply is required. Fortunately these are still available from several suppliers, ie Farnell 1516410 for £39.33 each. See Repair HP 8657D Signal Generator for details. I don't have the facilities to test the Modulator although it powers up ok with another power supply. 
The DQPSK I/Q Modulator Unit has now been SOLD (thanks Tsvetelin)