[atari7800] 512K Carts in Mess ?

Me, again, probably this is painfully obvious, but I can't see..

How on earth can I get a 512K cart to boot in MESS ?

I'm trying something along the lines of this cart configuration:
http://www.atarihq.com/danb/7800cart/C301105.shtml

512K ROM, Pokey, 32 banks of 16K..

The header A78 header is setup correctly, yet MESS just dies on booting up
with an illegal operation ? 

The header looks like this, which I think is correct ?
I'm suspecting that it's one of the (Special bits) 8-15 in the cart type
field ? But I've no idea what to plug in there for the above 512K
configuration ?
 
HEADER:
        DC.B    1                       ; 0       Header version     - 1
byte
        DC.B    "ATARI7800"             ; 1..16  "ATARI7800       "  - 16
bytes
        DS      7,32                    ;
        DC.B    "Test 23.09.04" ; 17..48 Cart title          - 32 bytes
        DS      HEADER+49-.,0   ;
        DC.B    $00,$08,$00,$00 ; 49..52 data length         - 4 bytes
        DC.B    $00,$03         ; 53..54 cart type           - 2 bytes
                                        ;    bit 0 - Pokey cart
                                        ;    bit 1 - Supercart bank switched
                                        ;    bit 2 - Supercart RAM at $4000
                                        ;    bit 3 - ROM at $4000
                                        ;    bit 4 - Bank 6 at $4000
                                        ;    bit 8-15 - Special
                                        ;         0 = Normal cart
        DC.B    1                       ; 55      controller 1 type  - 1
byte
        DC.B    1                       ; 56      controller 2 type  - 1
byte
                                        ;    0 = None
                                        ;    1 = Joystick
                                        ;    2 = Light Gun
        DC.B    0                       ; 57 0 = NTSC 1 = PAL
        ORG     HEADER+100              ; 100..127 "ACTUAL CART DATA STARTS
HERE" - 28 bytes
        DC.B    "ACTUAL CART DATA STARTS HERE"

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