The original manuscript is extant here and reads “the dark veil of unbelief being cast away from his mind”. In agreement with the was in the preceding clause (“for he knew that king Lamoni was under the power of God”), the 1906 LDS large-print edition supplied the expected finite verb form was, which was then followed in the third printing (in 1907) of the 1905 LDS missionary edition as well as in all subsequent LDS editions (from 1911 on). The RLDS text has retained the earliest reading with its nonfinite clause form.
If an original was was lost from the text, it must have occurred as Joseph Smith was dictating the text since, as noted above, 𝓞 is extant here and lacks the was. To be sure, a was could have been accidentally omitted during the early transmission of the text, as in the following list of initial errors in the two manuscripts:
There are no other examples of “to know that ” in the text (original or otherwise), but there are examples in the original text of “to know that ”; in each case the infinitive form involves the verb be (and is thus similar in that respect to the non- finite form being in Alma 19:6):
In the editing for the 1837 edition, the to be in 1 Nephi 1:3 was replaced by is, while in Moroni 4:1 the that was deleted. We should also note that there are examples in the text of present participial clauses in subordinate contexts that are never completed (see the examples listed under Alma 16:21). Thus the earliest reading here in Alma 19:6 may be intended.
Ultimately, it is difficult to decide the original reading here. But since there are two similar examples in the original text of nonfinite be used in the same context of “to know that ”, the critical text will accept the earlier but difficult reading in Alma 19:6: “he knew that the dark veil of unbelief being cast away from his mind”. Here we may have a case of a Hebrewlike present participial clause where the finite form of the be verb (here was) is understood. For other possible examples of unattached present participial clauses in the original text, see under 1 Nephi 3:17. Of course, the possibility remains that an original was was accidentally omitted from the subordinate clause in this sentence.
Summary: Accept in Alma 19:6 the earliest reading without the finite verb form was (“he knew that the dark veil of unbelief being cast away from his mind”); despite the difficulty and uniqueness of this present participial construction without the was, the original text has similar examples involving the infinitival form to be.