The rest of the story can be found at http://sliderulemuseum.com/Calculators.htm :
Slide Rule Calculators are defined as electronic calculators having the same functions as found on a Basic slide rule with A,B,C and D scales, minimally a multiply, divide and a square root key. Add in square, inverse (CI scale), and Sin, Cos and Tan trig functions and you have what was on Mannheim slide rules defined in 1850. This differentiates them from the four-function (+,-,x,/) calculators first introduced in 1970. Between 1970 and 1972 there were about 100 models of 4-function 'pocket' (some would be considered portable at least) calculators being manufactured or rebranded for the consumer market. When the slide rule calculator was first developed and made available to the common public in 1972, it was only natural to name these very pricey marvels for the tools they were about to replace.
From the same webpage:
This is the defining moment of the death of the slide rule. When the TI-30 was released it was the same price as a high-end advanced slide rule like the K&E 4181-3, (the slide rule only has a total of 11 unique parts so this was amazing). Inside the TI-30 is identically to the SR-40, the massive cost reduction was the sum of two details: The rechargable battery pack was replaced by a simple 9V cell and the molded lettering of the coloured keys replaced by a printed key plate. Battery consumption of the TI-30 was horrible. Probably the best selling calculator ever with about 15,000,000 manufactured units between 1976 and 1983.'SR' prefix dropped in favor of 'TI'. This is one of the last TI calculators that mentions 'SLIDE RULE' in its literature.