It was his 1905 paper on special relativity, 'On the Electrodynamics of Moving Bodies'.
A. Einstein (1952), from a letter to the Michelson Commemorative Meeting of the Cleveland Physics Society, quoted by R.S. Shankland, Am. J. Phys., 32,
16 (1964), p35.There's an example to look at here if you scroll to the Minkowski diagrams at the bottom. The remainder of the page is also on the topic.
I'm not familiar with argument along the lines of 'the equations show two entities and therefore the two are exclusive and different'. I would put forth the entity a (a=F/m) and the entity dv/dt (dv/dt=ds2/d2t). I don't think there is anything preventing E and B from being two considerations of the same thing (e.g. if some 'higher' relationship constrained the two or if something GUT-esque came along) even though they are commonly shown by us now as two different equations in certain formations.
You gave a link, presumably to provide examples of magnetic fields originating from other than charge. Checking that link I saw no mention of any origins of any magnetic fields. Was that the link you intended to give.
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