A look at the internals of the Brunsviga Model A and Marchant EEG9 mechanical calculators, as well as a brief description of how pinwheel calculators work.

Brunsviga Model A introduced in 1894
Marchant EEG9 introduced in 1927 (one of the most advanced pinwheel calculators ever made after this Marchant switched to their silent speed proportional gear calculators. Other companies such as Hamann Manus made pinwheel calculators of similar complexity)