In the rear of this book are photos of the "F" and "H" models as well as several pictures of special configurations available. These are F-models with fractional 8ths, fractional 12ths, and Sterling (British money).
The section on "Engineering" is of particular interest. It covers the principal engineering disaplines of the day including Civil, Architectural, Mining and Surveying. The chapter includes tables and detailed instructions for all manner of calculations, as well as "Properties of Sections", "Strength of Materials" and "Figuring Stresses". All these calculations could be done on the Comptometer; truly remarkable for its day.
Depicted on page #238 is a machine configured especially for engineers with an 8ths column on the right for fractions of an inch and a 12ths column left of it for inches. The remainder of the keyboard appears to be standard for counting total feet.
This machine appears to be unqiue in its mix of features. It has the "notched" keystems and the full row of oil holes on the register cover, both first introduced with the F-model. However, it also has the unnumbered decimal indicators of the C-model AND the *white* Controlled Key found only on E-models!
The D-model was reported by McCarthy in the 1924 Digest as carrying serial numbers 49001 thru 49154 which indicated to me that this could have been a special order for a single customer around 1913 just when Felt was trying to launch the new E- and F-models to block the new competition from the Burroughs Calc recently introduced.
While the foregoing is certainly not proof that this was the missing 'D' machine, the time period and its obvious special configuration present the strongest evidence currently available.