The Glider by Stellio Frati from 1946 uses kg for force, which is OK for strength, but in fluid dynamics you have density as kg s^2/m^4, which is almost as confusing as slugs. Maybe Italy was slower to adopt, they had barely abandoned Roman numerals.Thank you and bonus points for being the first one to directly answer the question . Your experience is what I expected but was surprised to find the Pajno books published since 2016 to use the kgf (MKFS system) for force and kgf/mm^2 for stress. Perhaps it has more to do with the vinrage of the author . Good books though and worth reading. Any other with a European aircraft design background?