4] Here is the instruction page that is sent to dealers with the new Fatter
4] Agnus chips.

  
3] 8372 FATTER AGNUS INSTALLATION PROCEDURES

 The new 8372 FATTER AGNUS IC, (P/N 318069-02), will require the following
 modifications for proper operation.  If no modifications are performed the
 IC will address only 512K of Chip RAM and operate only in the NTSC Mode.

4] NOTE: The correct IC Extractor Tool must be used when removing the
4] original 8370 FAT AGNUS from the socket.  The Extractor Tool is
4] available from Commodore Parts.  P/N 314874-01

3] INSTALLING FATTER AGNUS IN THE AMIGA 2000
 1) Locate Jumper J101 (Lower Right of Power Connector CN400 and move the
    installed shorting block to the left, shorting pins 2 and 3.
    * This will enable Address 19 from the 68000.
    * See Page 13 of A2000 System Schematics
 2) Locate Jumper Pad J500 (Lower left of 8520 IC at U301) and use a
    sharp Exacto knife to cut the trace connecting the two (2) pads.
    * This controls the _EXRAM signal.
    * See Page 9 of A2000 System Schematics
 3) Line Pin 1 of the 8372 FATTER AGNUS with Pin 1 of the socket and
    insert IC.

3] INSTALLING FATTER AGNUS IN THE AMIGA 500
 1) Locate Jumper pad JP2 (Between 68000 at U1 and ROM at U6) and use a
    sharp Exacto Knife to cut the trace connecting the Bottom and Center
    Pads.  Solder the Top and Center Pads.
    * This will enable Address 19 from the 68000
    * See page 39 of A500 Service Manual
 2) Locate the RAM Expansion Bus Connector CNX.  Pins 1 and 2 are on the
    bottom row and pins 55 adn 56 on the top row.  Count up from the
    bottom, (by 2's) and locate pins 41 and 42.  About 1/8" to the left
    is a plated hole with a trace which runs straight up, (parallel with
    the white silkscreen line designating the CNX connector outline.) 
    This trace should connect between to pin 32 of CNX and pin 32 of the
    GARY IC U5.  This trace must be cut.
    * This controls the _EXRAM signal.
    * See pages 39 and 40 of A500 service manual
4]    NOTE:  There is also another trace running straight up next 
4]           to the trace which must be cut.  This trace may be 
4]           obscured by teh silkscreen line.
4]           BE VERY CAREFUL THIS TRACE IS NOT CUT
 3) Line Pin 1 of the 8372 FATTER AGNUS with Pin 1 of the Socket and
    insert the IC.