Nice and atmospheric photos of the 9F and the '61', and that is not graffiti on the wall.
Occasionally some photos include details which can be easily overlooked and yet are as much part of our railway history as the actual subject of the photo, the 9F and the '61' are examples of just that. There are stacks of firebricks against the shed wall - inside to protect the friable material from the weather. Each stack is underneath a letter chalked on the wall... those letters identify the shape and size of the brick so that the person responsible for building a brick arch can find the correct bricks for a specific class of engine. Somewhere here I have an official book of diagrams which shows the arch formation and details of the required bricks for each of the standard GWR classes.
There is a similar piece of railway history in the car park of the Swindon outlet centre. Tacked to the brick wall at the north end of the site are lots of rectangular pieces of plate with numbers and letters... I am pretty sure that this area was a stacking ground for raw materials and that those plates identified the steel sheets, with dimensions and classes, for making boilers and fireboxes.
regards, Graham