Peugeot 703? Do you mean 403 possibly? 703 is a relatively recent model. If you mean 403 then it is not one of those. Being Mali it could be another French car. I think it is likely a Simca Vedette of around 1956/7: https://www.jalopnik.com/the-simca-vedette-a-50s-american-style-sedan-built-in-1847...

I had a quick look on the circuit page but the tach gauge is driven by the central display module CPU

I’ve never dived into a driver display module or central display module ( I’m not sure what the official name is)

This is what you get in VIDA on the 99 v70, the 98 might be a bit different

I’m not sure where engine speed is fed into the display processor, somewhere on the left

So my choices are to leave as it is now, unhappy wife, or fit the 07 units with the blue caps not the grey.

There is no choice here , really, only the illusion of choice.

Send us a picture of the blue caps when done

30 seconds of propane heat on those flare nuts amd off they come

Maybe connect a scanner and try to read revs in software over OBD? That would confirm crank sensor and wiring are good up to the display unit?

I think you have isolated it to the display module!

These are 22 mm socket fits. The socket needs to be fairly thin walled to fit into the recess of the m56, my fat impact 22mm just squeaked into the switch enough to not strip.

If you are from Yemen or other legacy county, you can use try a 7/8 thin socket. Might be a bit tight

22mm deep socket needed for this removal. Non critical torque, just snug it up

My idler pulley on my RN engine failed after 30k miles and wiped out my new valve job. INA one, too.