Weighing in from the frozen North country...

I'm sitting here holding in my hot little wood-fired hands a pair of Volvo accessory Smiths tachs, one brand NOS 6cyl unit in original Volvo box, pt# 281441, and a second, used but functional, marked as "4cyl" on face...

They are identical cosmetically except for: 6cyl or 4cyl designation on face, "RVI#" (5610/00 vs 5413/00) on face, redline (5500-6500 vs 6000-7000 RPM), and marking intervals (every 500 RPM vs every 200 RPM).
Inside, they are essentially IDENTICAL electrically...

There is lotsa' info on these tachs online last time I checked (half a dog's year ago), including schematics, both from Volvo folks but especially British car fools, where used commonly...

Because these tachs are fundamentally electrically different from the much more common single-wire tachs (because electrically they live BETWEEN the coil and distributor, rather than NEXT TO the coil), they are indifferent to the number of cylinders, and can be used with ANY number of cylinders, including, I believe, odd numbers...

Both these tachs have displayed virtually identical numbers connected to my B18/20s, and can be fine-adjusted by turning the potentiometer (R4) that is easily accessible through the back. This can be done at any single reference RPM, and will be correct for that RPM...How accurate throughout the rest of the range will depend on how the electronics have "mellowed" over the decades.

So if you want it to work, just install it as intended (between coil and distrib)...If you want it to appear correct to the knowledgeable Volvo crowd, you will have to change the face...

Back to toasting marshmallows,

Michael