In that case i'd measure the hole and fit a tacho :
https://www.europaspares.com/catalog.../category/356/
https://www.europaspares.com/catalog.../category/356/
First is 80mm and second is 100mm diameter, both should be in keeping with the OE Volvo instruments (similar font on the dial), i'm guessing one or the other should fit straight in. Also guessing i'd say at some point the oiginal console was broken/damaged so a previous owner replaced it with whataver guage that was originally, found the guage didn't work and covered it over after stripping it just to stop things falling through the hole.