Cleaning Murray viewer

[supa8]

Hi
I have 2 Murray viewers, 1 x 8mm and 1 x 16mm
Does anyone know how to clean the inside mirrors?
I have managed to clean the prisms at the front, but accessing the inside mirrors seems rather tricky.
I took that 4 screws out that are on the side, hoping the back cover (under the screen) would be taken off easily but it isn't budging.
Any ideas?