DIMENSIONS: 25mm x 12mm x 35mm (Please make sure that this is the correct size battery as there are 2 sizes available see below)
(colour may vary)

The following Avo Model meter have been manufactured

Model 7-1936
Model 40-1940
Model 8-1951
8 Mk II-1956
8 Mk III-1965
8 Mk IV-1970
8 Mk V-1972 [new internal design]
8 Mk V1-1984
8 Mk VII-1992-2008 when production ceased

The batteries used in the the meters are as follows.

Later models use the BLR154 type, both are 15volt and rated at 40mAh.
BLR121 Height: 36mm. Width: 15mm. Length: 26 mm.ebay item number 260635338455
BLR154 Height: 34mm. Diam: 15mm. ebay item number 400435143828