Pre-paint markings.?

[Beau Bruce]

Hi all, please forgive me if I do the typical stuff up with uploading photos as it is my first time trying to upload them but here goes..

I have taken these photos in the hope someone can inform me as to what this marking is on my 12 cab F60L.? The ARN has told me only that it is a typical GS truck but I have never seen a marking like this before on a CMP, that appears to have been applied well before the normal kahaki and ARN was applied to the vehicle. At the moment it only appears to be on the one L/H side of the front nose. The red square measures roughly 4"x4" in size and has what looks like "HK" stencilled into it with the letters being the same color as the background behind the red square.

Is anyone able to inform me regarding this.?

Thanks in advance.