Jevons's own 1870 paper dates the Logic Piano's completion to before 16 October 1869 and its Royal Society reading to 20 January 1870

jevonslogic-pianologic-machineshistory-of-computingvictorian-scienceroyal-societyprimary-source

W. Stanley Jevons's paper "On the Mechanical Performance of Logical Inference" — Philosophical Transactions of the Royal Society of London, vol. 160 (1870), pp. 497–518 — carries its own dating in the header line as printed: "Received October 16, 1869, — Read January 20, 1870." "Read" is the Royal Society's own term for a paper's oral presentation to its Fellows, so this line is the primary-source anchor for the exhibition date claim-jevons-logic-piano-1869-mechanical-inference-machine had previously sourced only to Wikipedia — and it sharpens "1870" to an exact day.

The paper's §32, describing the machine's physical construction, independently supports the 1869 build date in Jevons's own words: "The Machine, which has been actually finished, is adapted to the solution of any problems not involving more than four distinct positive terms, indicated by A, B, C, D, with, of course, their corresponding negatives, a, b, c, d." Since the paper containing this sentence was received by the Society on 16 October 1869, the machine was complete by that date at the latest.

Notably, the paper's roughly 20 pages of technical description never use the word "piano" — the popular "Logic Piano" nickname is not Jevons's own coinage in this text; its origin is unconfirmed (see the capture's further leads).

This is Tier 1 primary sourcing for a claim claim-jevons-logic-piano-1869-mechanical-inference-machine previously carried at seedling on Tier 4 alone; see also claim-oxford-history-of-science-museum-catalog-dates-jevons-logic-machine-to-1869 for independent institutional corroboration, and observation-jevons-double-bridge-victorian-logic-machine-to-ai-energy for the wider Jevons thread.