There are 4 known styles of conversions. One done in Australia by the VW factory, one done in Germany by the VW factory and two different styles in South Africa, again by the factory. For whatever reason, each factory chose to have their own design, but all accomplished the same task, clean engine air.

German style

Air is ported from each side of the rear window through scoops with protective bars, then down thru the double walled panel to the belly where it follows a long rectangular tube back to the engine compartment.

Notice in pic 2 that VW used a stock rear cabin panel and screwed it to the inside to make the double wall thickness. Plastic plugs that normally cover the access holes for the screws on doors and cargo doors, were used to plug the 4 holes that would've served as gate latch bolt holes.

This German example is a 1961. Total number made unknown.

I have seen 3 of these examples so far still in existence.

germdust2.jpg (59622 bytes)

3 protection bars per side total. Air is taken in through the scoops.
germdust1.jpg (106002 bytes)
2 =>
Down thru the back wall which is 2 stock walls riveted back to back  to create the 'channel'.
germdust4.jpg (63196 bytes)
Then through the belly box.
germdust5.jpg (137464 bytes)
And into the engine compartment.
germdust3.jpg (94047 bytes)
No rear side vents needed.
ger004.jpg (73372 bytes)
Here is a pic of it during restoration with the bed removed.

germdustplate.jpg (55266 bytes)
004- dusty model
010- Dust filter for passenger compartment
070- hoops/canvas
371 - unknown
Notice that the M-code plate is calling out Hoops/Canvas. I wonder how they ever installed the canvas with those big scoops in the way?

Here is another example of the same conversion
Thanks to Jon furst for the pics
Mvc-004s.jpg (157273 bytes) Mvc-005s.jpg (155547 bytes) Mvc-006s.jpg (154113 bytes) Mvc-007s.jpg (153136 bytes) Mvc-008s.jpg (154780 bytes) Mvc-009s.jpg (152589 bytes)
Australian style
Offered as of February of 1964. Total number made unknown. Similar to above, except for the method used to get the air into the truck. The clean air is taken in thru special additional vents on the roof peak line. Then it is ported through a double walled enlarged roof to the back rear window wall.
Once there, its the same as the German model, down thru the double walled rear window wall. Then through the belly in the long rectangular tube, back into the engine compartment.
The white Australian example is a 1965. I have seen 6 of these examples so far still in existence.
scpeak.jpg (125609 bytes) ausbubbroof.jpg (131088 bytes) mcode4_small.jpg (1421 bytes) ausvent.jpg (107039 bytes)

65.004aus.jpg (19415 bytes)
South African style
This is the same idea as the german style conversion. This is the only truck like this I have seen. Production numbers unknown.
67puscoopback.jpg (47723 bytes) 67puscoopoff.jpg (55451 bytes) 67puincab.jpg (49562 bytes) 67puairductfront.jpg (48662 bytes) 67pufront.jpg (58827 bytes) 67pulhside.jpg (54085 bytes) 67punovents.jpg (46922 bytes) 67puscoop.jpg (40335 bytes) 67puscoopside.jpg (47939 bytes)67puscooploose1.jpg (55167 bytes) 67puscooploose2.jpg (54373 bytes) 
South African style 
All of the fresh air truck conversions shown above from each country are Single cabs except for this one. The air is ducted thru the inlets that are attached to the backside of the gates. If the gates are lowered, there are screens to protect the intake holes remaining in the bed. One of the interesting things about this conversion is they are all RHD DC's with suicide rear doors! I have seen 6 of these examples so far still in existence. Total production unknown.
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1961 rhd South African crew cab, with factory color coded interior vinyl/panels etc. Submitted by Rikki James