I found a wonderful book recently named "Sentinel" Patent Locomotives that covers much more than just Sentinel 7109. Although it was reprinted in 1974, my copy was the genuine article published by the Sentinel Waggon Works in 1931 as a kind of technical marketing catalogue for pre-war locomotives.
For example, here's a LNER Y3 type in drawing format.
And the Y1 type; however, the difference is not apparent except to those who looked underneath.
The Y3 had a gearbox below its engine to enable two levels of capability whereas the Y1 drove directly from the engine's crankshaft. The Y3 was thus the more flexible locomotive.