Meccano with a Difference (Engine)

Charles Babbage's "Difference Engine", the first analogue computer, in Meccano.

It would have been possible to make Meccano, a standard construction system of interchangeable parts, from about 1860 - when the first interchangeable part guns were made at Springfield Armory in the USA. It's no stretch to say that it would have been possible to perform codebreaking and ballistics computations at least 75 years before they were in reality, and resource allocation and business computation likewise.

This would have led to advances in both shipbuilding and aeronautics 25 years earlier, and even regular space travel by 1950.

UPDATE: I missed it, but the excellent Melbourne blog Tramtown had an article on this a few weeks ago, and an even earlier post from 2004 with lots more videos. Worth a visit!