Hi Lloyd,
The service manual can be downloaded free from the excellent Elektrotanya website here:
https://elektrotanya.com/avo_ea113_e.../download.html
It contains exploded diagrams and guidance on the positions of shims and spacers. The design requires the case to be clamped by the four screws without putting undue stress on the seal area between the case and the front panel. If incorrectly fitted, the corners of the case can be cracked, as my example shows
Good point David about RF immunity. I once wasted some time trying to trace the cause of drifting zero in another electronic voltmeter, only to discover that it was picking up the transmissions from the mobile phone in my pocket!