This may not be much help, but I would like to raise a flag or two. As a note, the only primary source I have for H series other than photos is an Ernst Leichsenring
Logbook entry for V89. Most of the other information came from items in Flugzeug magazine (1 & 2/90) and Jet & Prop (2 & 6/97).
First-I believe it was highly unlikely that Werknummer 430821 was the H-2 prototype. The Stammkennzeichen RJ+ON does not fit, and is for an early batch of Ju 88 G-1s which makes some sense. Unfortunately, I do not have the linking Werknummer sequence for those Ju 88s.
Second-Griehl’s Ju 88 book has Werknummer 430821 as the second H-1, V90. The Stammkennzeichen for Werknummer 430821 is probably RG+RQ.
Third- The Stammkennzeichen for Werknummer 430898 may be TX+OL.
If you do find more “H” info, I hope you will post it here and what source.
ArtieBob