|"Introduced in the mid-1930s, SNs were intermixed with J-models as orders entered production. Certainly an
unusual looking machine, it has sub-totaling capabilities similar in design to that of the
Mechanical Accountant, a machine designed by J.A.V.Turek around 1900 (according to Ernst
Martin). Turek was employed by Felt & Tarrant from about 1910 thru the 1930s and was
most likely the designer of the SuperTotalizer.
The machine pictured exhibits a strange "oozing" of the digits imprinted onto the light colored keytops. It seems that this problem affected only some machines produced in the 1930s and perhaps was due to a chemical interaction between the ink and the keytop material.
This model is sometimes referred to as "duplex" because of the sub-total and grand-total dialsets. Such reference should not be confused with the duplex feature introduced with the A-model which allowed simultaneous depression of keys and was a significant operational feature."
Source: The Comptometer Memorial Website