It is also interesting and noteworthy that this Commentary mentions dry insight workers (sukkhavipassaka):
“The Elder, excluding ordinary persons, stream-enterers, once-returners, non-returners, dry-insight practitioners, and many hundreds and thousands of monks who were not arahants, selected only those arahants who were masters of the entire ninefold Dispensation, bearers of the full range of the Tipiṭaka, attainers of the analytical knowledges (paṭisambhidā), of great power, mostly declared by the Blessed One as foremost in various categories (such as the threefold knowledge)—in total, four hundred and ninety-nine arahant monks.”
Thanks to @RobertK for providing the following link:
Here is an alternate translation from there:
The monks said, “Well then, venerable sir, let the elder select the monks.” The elder, versed in the entire ninefold dispensation of the Teacher, passed over many hundreds and many thousands of ordinary people, stream-enterers, once-returners, non-returners, bare-insight monks, and arahant monks, and selected only just under five hundred arahant monks who were masters of the entire Tipiṭaka with its divisions of learning, attained to the discriminations, were of great power, and were mostly designated by the Blessed One as foremost in such categories as those possessing the threefold knowledge. Referring to this, it was said: “Then the venerable Mahākassapa selected four hundred and ninety-nine arahants” (Cūḷavagga 437).
Why then did the Elder leave out one? To grant an opportunity to Venerable Ānanda. For with that venerable one, or without him, it was not possible to hold the Dhamma recitation. That venerable one was a learner and needed training, therefore it was not possible with him; moreover, since he had not personally received any of the teachings declared by the Ten-Powered One—such as the discourses and verses—directly from the Blessed One, it was not possible without him. If that were the case, even though he was a learner, due to his great assistance in the Dhamma recitation, he should have been selected by the Elder. But why was he not selected? To avoid others’ criticism. The Elder had great confidence in Venerable Ānanda, so much so that even when gray hairs appeared on his head, he admonished him with the words, “This young man does not know moderation.” Moreover, this venerable one was born into the Sakyan clan and was the Tathāgata’s brother, the son of a paternal uncle. There, the monks, thinking it was a matter of favoritism, might have criticized, “The Elder selected Ānanda, a learner who had attained analytical knowledges, while setting aside many monks who were arahants who had attained analytical knowledges.” To avoid such criticism, he did not select him, thinking, “Without Ānanda, the recitation cannot be done; I will take the consent of the monks.”
The whole Commentary can be read there.
Renaldo