From the Fastenal website, 2 contenders:

TM2550016A20000  M6-1.0 x 16mm S/S A2 Slotted Truss Head Machine Screw
$0.6517 each

TM2550020A20000 M6-1.0 x 20mm S/S A2 Slotted Truss Head Machine Screw
$0.6025 each

both have the pitch size of 1 mm, which i believe is correct.

The stock screw from the Van measured 20 mm overall, the measurements
above are from base of head to end of screw.

Long URL to particular page, the two above are at the bottom:

http://www.fastenal.com/web/products.ex?
c=600059&s=m6&dispatch=search&curPage=6&page=6&next=Next

click on the links for each screw to get more details, ie:

"Truss Head Screws are also knows as oven head or stove head screws,
Truss Head Screws have the largest head diameter of all machine screw
head styles.
Ideal for use in enlarged holes.
Truss Heads have a low, rounded top with flat bearing surface, length
is measured from under the head."

They appear to be quite similar to what GoWesty calls carriage bolts :)

Alistair

'82 westy, diesel converted to gas in '94
http://www.members.shaw.ca/albell/