Another variation involved adding a "sub-cents" column to the right of the normal ¢¢ columns. Indeed, Felt's defining patent issued in 1887 illustrated just such a layout with keys labeled 1/8 thru 7/8. The F-142337 (an early 12-column machine) has 7 keys marked simply as 6-1 thru 0-7. And F-115314 sports an extra column of 11 keys apparantly for calculating inches to feet. Yet another variation, named the "Engineer's" model was shown in Applied Mechanical Arithmetic with 8ths and 12ths columns on the right.
All early machines made for export to the British Commonwealth of nations for calculating Sterling currency were required to count pence to 12 before carrying to shillings which then had to count 20 before carrying over to £ . Some even had an extra righthand column for counting farthings (4 to the penny).
In all cases where a column was other than 9 rows, the internal engineering had to be modified to allow the keystroke to drive the register wheel thru the proper arc. For 7 rows (8ths), this would be 1/8th of a full rotation and for 11 rows (12ths), 1/12th of a full rotation. And, of course, the numerals on the register wheel had to reflect the changes. Such customizing could easily have doubled the machine's price unless a large order was involved.
Clearly, the vast majority of Comptometers were "run of the mill". It is not known just how common it was to produce custom configurations on special order as few other than mentioned above have come to light..
Note the layout with alternating 3-column sets of white and pale blue keytops and a single "billions" far-left column. A reasonable guess would be that this was a special order for one of the world's single-value currencies. Or, perhaps some non-monetary use.
This rough example was recently unearthed in Adelaide, Australia. Look closely and you can see that the rightmost column had 3 keys for calculating in farthings (4 per penny) while most later machines did not (the inflation effect?). Note that the "carry over" from the shillings columns (3rd & 4th rightmost - 20 per £) was the equivalent of a carry from ¢ to $.
This earliest of the monster Compts (wgt:29 lbs!) must have been destined for a government office as almost no private businesses of the day had accounts which would reach into the billions, altho such money magnitudes seem common today.
A sharp-eyed longtime Compt collector, Bob De Cesaris, has pointed out that the keyplate and possibly the case, seems to have been made in two parts. Considering the bulk and heft of 12 column machines and this one's early date, Felt seems to have opted for this cost-effective make-do solution.
Felt used a clever trick here to divide the keyboard into a left and right side analogous to the traditional debit-credit sides of a General Ledger. It seems likely that the idea, while technically workable, resulted in no sales for that purpose since the machine would have been too expensive for any business whose general accounts would never exceed the $9,999.99 limit imposed on the credits side of the ledger. This particular machine is not known to have survived.
This particular machine did not calculate in farthings however, it had 3 higher order columns than the earlier B-model Sterling machine (see above) and therefore, was basicly a 12-column machine.
This machine could facilitate calculations where the lowest order had a 12-base and higher orders only the more common 10-base numbering. Feet/inches was the likely, but by no means, only use. Note how the extra keys had to be "stacked" above the normal 9, a solution that would also accomodate the exported Sterling machines requiring 12 pence to the shilling.
Note the 7-row rightmost column, (used for 8ths of what?).
While Sterling keyboards were relatively common in the Commonwealth countries, this machine is notable because it has been re-keyed with a set of the later model extruded (99C) keytops. Note also the "modern" clearing handle on this "shoebox" machine.