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The 123GT tach was a universal Smith's unit with a Volvo logo on the face.
For pure authenticity, you may find one at a swap meet. However, functionally, the 4 cyl Smith's unit will work. If you look up under the centre of the dash about 6" from the windshield, there may be a hole(about 3/4") in the steel where the OE tach mounts. We installed them as a dealer installed accessory, but I'm not sure if all 122 had the hole.
As George noted, the tach is really not needed. When the car stops accelerating, change up to the next gear. The advertising tag line for Canada in the '60s was "Drive it like you hate it".
My friend used to run his father's 90HP B18 down steep hills in 2nd gear up to incredible RPM. We did manage to break a pushrod once, but the car lasted for over 200,000 miles. World's Fastest Tractors!!!