SR 51Model: SR-51 1v
Display: 10 + 2
Battery: BP 1 
Adapter: AC9200
Year: 1975
Chip: TMC0501, TMS0522, TMC0523

With the SR-51 Texas Instruments began a series of scientific calculators that added statistical functions and conversions to the "usual functions". But none of them are engineered as flawlessly as the SR-51. It was built as a tank to survive decades of use like its close relative SR-50. Compared to previous models, the SR-51 got a keyboard design with white, gray, yellow and orange keys and gave a perspective to later TI calculators. 
To reduce manufacturing costs and to give a similar look and feel to the SR-52 and SR-56 calculators, the SR-51 was replaced within months with the SR-51A. Logically it is quite rare. However, two different variants are known: this model from an early production batch uses huge yellow characters for the 2 shifted functions, while the later SR-51 sports much finer print.

SR 0051 1v 01

SR 0051 1v 04SR 0051 1v 03SR 0051 1v 02

This rare model had many parts corroded by battery acid, the 7-segment displays completely corroded, the replacement and cleaning of the keyboard can be seen on the side.