Re: the heavenly witnesses: Codex Teplensis et al
Answering my own question, Codex Teplensis reads something like (I'm no expert in deciphering Fraktur):
. . . daz Krist ist de marheit. Wan dren sint di genent gezug im himel: der Vater, daz Wort, und der heilige Geist und dise drei sint ain und dry sint di deben gezeug uff der ereden: Geist Wasser und plut; und dese drey sin ann. Id wir enphachen . . .
This answers exactly to a ms of the Vulgate owned and cited by Adam Clarke.
Luther's version, on the other hand, answers, at least in v. 7-8, to the text of Bede's commentary:
. . . quoniam Christus est veritas. Quoniam tres sunt qui testimonium dant in terra spiritus aqua et sanguis et tres unum sunt. Si testimonium . . .
Isaac Newton wrote that 'in terra' was, as far as he could tell, added in a second hand to the lemma. There was, however, in the Harleian Collection of the British Museum, as described by Samuel Johnson in The Gentleman's Magazine (1814), a 13th-cent whole Bible Vulgate ms and an Old Latin ms of about the 9th century that both answer to this text. This text would result from taking the text as found in Codex Teplensis and deleting the first set of witnesses, on the assumption that ity was placed there by way of interpolation--without noticing that 'in terra' in the second set was also part of the original interpolation.
Thus we would have to conclude that even Luther's Bible, at least in the final edition that he edited, contained an interpolation from the Latin into the translation of Erasmus' first edition of 1 John 5:7-8.