Lorry, 4-ton, APF and Lorry, 4-ton, ZPI - Details needed

[Joe C]

I'm looking help digging out proper identification, more information, details and/or pictures of "Lorries, 4-ton, APF" and "Lorries, 4-ton, ZPI". These vehicles appear in the list of current vehicles in Canadian Army Overseas Routine Order 2872, 11 December 1942 but I haven't been able to dig up details on them.

As background, APF and ZPI were radar systems in the GL III(c) gunlaying radar used for air defence. The Lorries in question were the movers for the trailers. I have photos of what I believe are the two vehicles [1][2] taken from a series taken 26 Jan 1943 of 1 Radiolocation Unit [3] and so I think it's a good assumption they are of the named vehicles.

So, specifically, my questions would be:
- What make/model of Lorry are they?
- To me they look like "stock" lorries, what customizations made them 'unique' enough to give a name to them? As they had two names, I'd infer they had two different configurations/designs.
- Does anyone have other period photos of these lorries to share?

Anything would be welcome!

[1] http://www.rcsigs.ca/index.php/Lorry_4-ton_ZPI
[2] http://www.rcsigs.ca/index.php/Lorry_4-ton_APF
[3] http://www.rcsigs.ca/index.php/Radio..._Unit_GL_III(c)