See back wall for family history of these machines.
The Sears adding machine can only add and subtract directly. You can multiply by manual repeated adds and shifts. It also can't display a negative result - you will get the 10's complement of the result.
The Underwood Sundstrand has a 10-key keypad which allows a skilled operator to touch type so is faster to use. It also can only add and subtract directly. Multiply is a little simpler, and it can also properly print negative totals.
The Sears adding machine is model 187-580140. It was made by Remington Office Machines division of Sperry Rand Corporation. The model was originally made by Clary Corporation which was bought by Sperry-Rand in August, 1960. The Sears mechanism above took a lot of work to get working again. Sitting all these years unused allowed the oil to harden to a very effective glue. Some parts I could disassemble, clean and re-lubricate but others were installed with pressed/peened pins. For those I used penetrating oil and moved them when I could. Others such as the many rollers and gears I had to heat with my hot air rework tool (set to 250-300F) to soften the hardened oil enough to be able to move the parts and get new oil in. Without proper cleaning it is likely it will get stiff again if it sits too long. It also needed a new ribbon.
The Underwood Sundstrand had been well oiled during its life and
I only found a few parts stuck when I re-lubricated it. The ribbon was still
usable. The Sundstrand adding machine rights were sold to Underwood-Elliot
Fisher Company in 1927.