Markings and Graffiti

[John Ganton]

The picture shows the marking on the left rear of my 42 HUP. I understand the background indicates the RCEME. Can anyone identify the unit corresponding to the 15? The marking on the front left is like looking at shadows with no colours remaining but the number is clearly 75 and not 15.
The door number is Z58 - 700. Does this mean it was a home front truck?
Crudely scratched on the inside is the number 153624. If this is a service number is there any way of identifying the individual?
Thanks John