I would guess that is a rare find too. I have a Firenza Sport SL owner's manual which is quite rare as well.
I would be cautious about the oils that are 'recommended' in the handbook. The recommended oil viscosity is directly related to the average temperature of the location the car is going to be used in, the expected coolant temperature (thermostat) and the driving conditions. So, in a Northern European winter on country roads you can get away with a lighter oil such as 10/30 or 10/40 but in warmer locations, with a lot of motorway driving a 20/50 would be a better choice.
It is also worth noting that the book was published in 1968, the very early years of the Slant 4, and subsequent experience has shown that the high compression Slant 4 really puts a lot of heat into the oil and oil temperatures can climb a lot higher that the original design temperatures.
Compound that with the fact that the mineral oils of the past are not the same as today's mineral oil equivalents. Largely due to environmental and emissions restraints, they no longer contain the additive types that gave them their very high temperature stability and the cold start protection that the engine was designed for back in the 60's. Today we find that a modern 20/50 mineral oil will struggle in protecting the average high compression Slant 4.
Have a read at this post on oil types for more information.
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"When you have a Firenza like mine, who needs any Minis?" copyright David Maxwell 1979.