Puzzled by Beza's note, I contacted Jan Krans and asked him about it. Here's what he said:
" I will have a new book this year, with among other things an update on the Beza part, and a lengthy treatment of this case [in Rev. 16:5] in particular. Shortest version: Beza states that he had a manuscript, but wrongly so, because he misinterpreted one of his own handwritten notes. It is therefore a conjecture, by Beza, but he would probably not have changed his text, had he not misunderstood his notes. In that case he would also have written a different annotation, probably with the conjecture as a suggestion only."