Fordson 15-cwt with loudspeakers

Hello all!

My name is Frank, and I am new to the forum. I am a scale modeller and I am currently building Fordson 15-cwt as in the title. Photo that I am trying to recreate is from Normandy and most likely shows Polish soldiers resting after closing Falaise gap.(photo 1) Polish Armored Division didn't have such vehicles, so I thought it comes from some unit from 1st Canadian Army, or 2nd Canadian Corp unit, but couldn't find any matching unit. Then I read that 21st Army Group had 5 Amplifier Units. I looked for 21st Army Group Arm of Service numbers, but found only arlillery units chart done by "UK Tank Markings" site. Then I found photo from Westertimke P.O.W. camp liberated by Guards Armored Division and on this photo there was almost exact vehicle! Just door was a bit different, but then I was sure it is from the same unit. (photo 2) Also on other photos you can see Bedfords with 21st Army Group markings (photo 3), but again, I didn't find any Arm of Service numbers for 21st Army Group units. I know it is not 14th Amplifier Unit, as it was in Belsen camp with 11th Armored at this time.

So my question is: Can anyone provide me with 21st Army Group level units Arm of Service markings? What markings were on Amplifier Units vehicles? I was also considering buying Warpaint Vol.3 by Dick Taylor, but I don't know if I can find my answer there.

I would be very thankful for any reply!