This pre-dates the first hand calculator I know of, the early HP's from 1968. These machines used the same notation as the HP-35 pictured below (https://www.hpmuseum.org/rpnvers.htm#num).
The HP-35 calculator came out in 1972. According to the page below, the scientific notation used is of the form
1.234-8
i.e., without the E (see the second row of the gallery).
My (c) 1977 version of K&R C explains printf() around page 145, and it supports the %f notation for output of float's as 1.234e-8, etc.
This Fortran IV manual (early-mid 1960s, https://www.math-cs.gordon.edu/courses/cs323/FORTRAN/fortran...) lists the same exponential notation, but with capital E for REAL's and capital D for double's.
This pre-dates the first hand calculator I know of, the early HP's from 1968. These machines used the same notation as the HP-35 pictured below (https://www.hpmuseum.org/rpnvers.htm#num).
The HP-35 calculator came out in 1972. According to the page below, the scientific notation used is of the form
1.234-8
i.e., without the E (see the second row of the gallery).
https://vintagecalc.com/hp-35-red-dot/
Just a couple of data points.