I'm writing a paper on philosophical egology, and I kinda got stuck on Husserl.

Up until then, especially in German Idealism, they use the terminology of the 'I', pure 'I', transcendental 'I' (in German 'Ich' and 'das Ich'), etc. Kant, Fichte, Max Stirner... even Freud doesn't use ego, but 'Das Ich und das Es', translated as 'The Ego and the Id'. Only two cases that I found of original usage of term ego - Descartes, obviously, because of ego cogito, and in Husserl.

As far as my research led me, I found out that he starts using it somewhere around 'First philosophy' (1923-1924), and after. Especially in 'Cartesian meditations'. Before that, he only used 'Ich'. Also, there is an interesting paper (to which I, unfortunately, don't have access), that 'pure I' and 'transcendental ego' are actually quite different notions in later Husserlian phenomenology. (Joseph Kockelmans, "Husserl and Kant on the Pure Ego", in Husserl: Expositions and Appraisals, ed. F.
Elliston. Notre Dame: University of Notre Dame Press, 1977).

For example, in 'Cartesian meditations', §33, Husserl says:

"From the Ego [in original: Ich] as identical pole, and as substrate of habitualities, we distinguish the ego [also 'ego' in original] taken in full concreteness - in that we take, in addition, that without which the Ego [original: das Ich] cannot after all be concrete. (The ego, taken in full concreteness, we propose to call by the Leibnizian name: monad.)"

(This is Dorion Cairns translation, 1960).

There are numerous passages like this, "I exist for myself... This is true of the transcendental ego... of the psychologically pure ego..." etc. You get the picture.

Does anyone have any idea or lead to this difference? It's obviously not just terminological, or synonyms?