Small tachs mostly appeared in 240 GLT and GT models. I'm guessing the small tach with the higher red line is possibly from an early 240 GLT with a B23E engine and the lower one possibly from a later GT turbo. Could also be block differences like B21F/FT vs. B230F/FT. Peak rpms are notably different. Reference this old post here regarding large tach differences https://www.brickboard.com/RWD/volvo/319267/220/240/260/280/re_stock_redline_b230f.html
Other red line differences could for the various 2.0 litre blocks used in Europe.
Do a Volvo parts lookup using the p/n's off the back then check the fitments listed for it. Some of the fitments may not be as engine specific as you'd like, but if you come at them a number of ways such as which engines were available in which years or doing p/n lookups on different sites (like Skandix) then you can usually figure it out.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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