The OA-3633() consists of the AM-2060() Audio Amplifier/Power Supply and the CX-4655 Cable.   The "A" version has some extra power supply filtering, probably to increase it's TEMPEST rating for use with COMSEC equipment.

AM-2060() Audio Amplifier / Power Supply

The PRC-25 or PRC-77 Radio Sets can be used in either a man pack (GRC-125, GRC-160) or vehicle (VRC-53, VRC-64) configuration.  When used in the vehicle configuration the Radio Set is installed into the AM-2060() Audio Amplifier/Power Supply.  TheAM-2060() Audio Amplifier/Power Supply is installed into the MT-1029 Mount which provides the 24 Volt DC power input, interface to the VIC-1 Vehicle Inter Communications system, or COMSEC equipment like the KY-57 or KY-38 by means of the J-3513 Junction Box.

The AM-1777 looks very much like this, how is it different? Let me know. One thing is that the AM-1777 only works with the PRC-77 (it does not have the low voltages needed for the PRC-25).

CX-4655 Cable

The cable connects between the AM-2060 and the Radio Set.  It provides power to the Radio Set and gets the line level audio from the radio to feed the audio amplifier and drive the speaker.  The CX-4655 comes in either a 9" or 11" long version.  They both have U-316 connectors on each end since the Radio Set POWER connector is identical to the AM-2060() Radio Set connector.  The cable has all the pins wired 1:1.  Note when this cable is used the DC vehicle power is fed to the MT-1029, which feeds the AM-2060 Power Supply which in turn feeds the PRC-25 or PRC-77 radio, therefore the radio's internal battery is not being used.

Note you can use a couple of my Power Connector Kit (PCK) to make up a cable since it's the connector the mates to the POWER connector on the PRC-25/PRC-77 and the connector on the AM-2060.

Power Supply

The PRC-25 and PRC-77 Radio Sets operate from standard "12 Volt" civilian vehicle DC power, like you would get from a Cigarette Lighter Socket.  They can be powered using the VPA-2577 Vehicle Power Adapter or 257477BA Battery Adapter.  But when used in a military vehicle with "24 Volt" power, a power supply is needed to supply not only the "12 Volt" main power but also the 2.5 Volt DC for various parts of the PRC-25 (the PRC-77 does not need this supply).  The AM-2060 converts the military vehicle 24 VDC supply to the various voltages needed by a PRC-25 (or just the 12 V needed by the PRC-77).

Audio Amplifier & Speaker

The background noise level in a military vehicle is higher than that experienced by a man on foot, so an audio amplifier and speaker allows the driver and/or passengers to hear the radio in a hands free manner.

Note that there is no RF connection between the AM-2060 and the Radio Set or Antenna.  There is no RF amplification involved with the AM-2060.  The OF-185 contains both an Audio Amplifier/Power Supply and an RF Power Amplifier for the PRC-68 Family of Squad Radios.

VIC-1 Interconnections

The VIC-1 system allows crew members to hear the radio and in some cases transmit on the radio.  In addition the radio can be interfaced into a retransmission system either to a radio in the same band or to a radio on another band.  For example a PRC-77 + AM-2060 + MT-1029 and be connected to a C-2299 Radio Relay box and then to a GRC-213 H.F. radio to make a cross band repeater.  For this application the retransmission function is handled by the AM-2060 and MT-1029, not the AUDIO connector on the PRC-77.

Antenna Band Control

The AS-1729 Antenna System uses the MX-6707 Base Matching Unit which needs to be set for the frequency band in use.  This can be done using the manual band switch or remotely using electrical signals.  These can come from the RT-246, RT-524 or the AM-2060.  The AM-2060() has a band switch on the front panel that needs to be set for the frequency band that includes the operating frequency of the Radio Set.

Alignment Pins

The AM-2060A is located by the MT-1029 using two alignment pins that are a close fit.  My AM-2060A would not go onto the pins until I loosened the nut on the MT-1029 pin that's on the junction box.  I also applies a film of Radio Shack Lube-Gel to both pins so there will not be any corrosion.

Order of Cable Connection

I have the radio system indoors, not in a vehicle.  It's best to connect all the cables to the MT-1029 BEFORE adding the AM2060A and PRC-77.  Once you've got all the radio equipment attached it's more difficult to install the cables onto the MT-1029.

Rotating Connector Keys

One of the connectors coming from the J-3513 had it's connector Key rotated from where it needed to be.  You can use pliers to grab the straight knurled sleeve behind the connector and turn the Key to line up with the equipment.

The MWM type vehicle connectors used for DC power in and out as well as for the VIC-1 interfaces have recessed nuts and you need a spanner to remove both nuts so you can slip out and reinstall the connector proper in at any 45 degree increment.  This is often needed to get the cables to fit.
I got a set of spanner wrenches from Fair Radio and a couple of them are just the right size for doing this.

TM 11-5820-498-35 DS, GS & Depot Maint Man Amplifier-Power Supply Groups OA-366()/GRC
This manual is part of the VRC-53/GRC-125 for the PRC-25 and VRC-64/GRC-160 for the PRC-77 -498 series of manuals.
It includes operation with VIC-1 Vehicle Intercom equipment since the AM-2060 interfaces to the VIC-1 through the MT-1029 Mount.