Karl Korsch group's journal Kommunistische Politik: Diskussionsblatt der Linken appeared in 1926-27. It's not online, except for a separate brochure which contained reprints of its pieces on the Russian question. It contains also the Russia section from Bordiga's Lyons Theses (which had appeared in French).
But aside from Korsch's own writings, it would seem that among the contributors to this journal was Boris Roninger (also spelled Roniger), who, it so happens to be, was a staunch adherent of Luxemburg's theory (as evidenced in his critical article on the eve of the 5th Comintern congress in 1924).
If anyone is able to check a library/archive, it would be good to go through this journal, to see if Roniger wrote anything further in it about economics.
Though his group published eg the Russian Decist opposition platform, the Korsch group usually is not classified as leftcom. But I don't know if the same can be said about Roniger. He was probably more to the left than Korsch (at least Bukharin responded to his 1924 criticism by tagging Roniger as a leftcom).
What happened to Roniger afterwards? Well, his big book on Trustkapitalismus was scheduled to appear in the same series as Henryk Grossman's, but it never materialised and the manuscript, if it survived, now seems difficult to locate.
He tried to go America (enlisting the help of Einstein for a grant) without success, so fled to Switzerland, where he probably learned about Swiss liberal theology, eg that of the famous Karl Barth, or Fritz Lieb, a Slavist theologian, who in early 1920s had been sympathetic to communism. After the war Roniger became professor in Oxford, apparently in theology (eg a lecture on "Karl Barth's Philosophy of Religion”, Oriel College), but he still was leftwing.
Roniger was born in 1896 in Odessa, later emigrating to the West. It's not a specifically Jewish name, so I suppose he could be a Russian of German origin. I'm not even sure it was his real birthname (could be he adopted it later). The date of his death is unknown to me.