This is my original answer posted on Sept. 13, 2020, emphasizing the lack of mutual acknowledgement of the two prominent contemporaneous 'electricians' Steinmetz and Heaviside on opposing sides of the Lake. I've decided to resurrect it after reading Kline.
Read "Steinmetz and the Concept of Phasor: A Forgotten Story" by A. E. A. Araújo & D. A. V. Tonidandel.
Berg, Carslaw, and Carson were colleagues of Steinmetz and promoted through various publications (near or after the death of Steinmetz, perhaps tactfully) the operational calculus of Heaviside, based on the Laplace (and Mellin transform), which can handle causal and transient electric circuit phenomena that the Fourier transform can't, yet Steinmetz never mentioned Heaviside nor maybe even the Laplace or Mellin transform in his publications that I have perused (he does, Fourier series and harmonics)—whether through ignorance, intentional neglect, or lack of necessity I don't know.
Steinmetz—The Forger of Thunderbolts, The Wizard of Schenectady—is a fascinating character, living and working in the Silicon Valley of the times—Electric City (Schenectady, New York, circa 1890s)—ushering in the age of electricity with other luminaries such as Edison, Westinghouse, Tesla, Marconi, and Einstein (don't forget Einstein got the Nobel for the photoelectric effect and there is a photo of him posing right next to Steinmetz at a Marconi wireless station).
I'm fairly sure Steinmetz developed his mathematical approach independently of the others I've mentioned. Proteus (Steinmetz) was regarded as a demigod at G.E. (vetting all new ideas) and was promoted as such by the astute P.R. of G.E. (they even "photoshopped" the photo I mentioned to remove everyone but Steinmetz and Einstein), but I don't recall, from my scannings of their work, him or his associates Carslaw or Carson referring to a Steinmetz transform. Perhaps the phasor concept comes closest to fitting the bill, or the Steinmetz circuit was translated as the Steinmetz transform.
A great documentary, free until Dec 2020: Divine Discontent — Charles Proteus Steinmetz.