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Karl GrΓΌn and Moses Hess.
Engels appended a footnote to that section in 1890. It reads:
The revolutionary storm of 1848 swept away this whole shabby tendency and cured its protagonists of the desire to dabble further in Socialism. The chief representative and classical type of this tendency is Herr Karl GrΓΌn.
Wikipedia says that Karl GrΓΌn was "associated with the group of 'True Socialists' around Moses Hess, a Young Hegelian philosopher and forerunner of labour Zionism."
So look for people associated with those two, and those are your "True Socialists."