Thanks to you both Jack and Gadge for your replies.

After posting my request I spent a lot of time trawling the forum, looking for a similar baseplate. A 1978 utility owned by forum member Cuscus looks like my photo and also fits with your knowledgeable replies.

https://www.outdoorking-forum.com.au/forum/ubbthreads.php/topics/34652/victa-utility-identification.html

Thanks again