Dual or Vinaya Idiom?

Dear Pali Group,
I came accross an interesting grammatical problem.
There is a difference in the wording of two related passages regarding
the carrying out of the the ~natti, motion, of the Invitation,
pavaara.naa, in the Mahaavagga:
In the case of 4 monks in a residence a senior monk has to address the
other 3 with: "Su.nantu me aayasmanto, ajja pavaara.naa.” However in the
case of 3 monks he has to address the other 2 with: "Su.nantu me
aayasmantaa, ajja pavaara.naa.” (Ee M I 162/Mv 4.5, BJ p. 409-10)
The form aayasmantaa is also found in a few other passages relating to
Sanghakammas,e.g., M I 124. In the Paatimokkha, etc, addressed to a
Sangha, only the form aayasmanto is found.
Why is the form aayasmantaa used instead of aayasmanto? Both are
addressed to more than one monk and I don't see a clear reason why
aayasmanto should become aayasmantaa.
Aggava.msa also noticed it in the Saddaniiti and categorically refuses
to take it as an example of the dual in Pali. I made a tentative
translation of his arguments  which I reproduce below.
Maybe some members of the group have something to say about the matter.
Is it an example of the dual or is it just a Vinaya idiom or is there
some other reason? It might be of interest to check the
Muulasarvastivaadin Pravaara.naa karmavaacaa and see whether there is a
distinction in their Vinaya too. I don't have the edition of their
Vinaya though.
Best wishes,
Bh. Nyanatusita


Saddaniiti Myanmar p. 123:

Kasmaa pana imasmi.m pakara.ne dvivacana.m na vuttanti? Yasmaa
buddhavacane dvivacana.m naama natthi, tasmaa na vuttanti. Nanu
buddhavacane vacanattaya.m atthi, tathaa hi "aayasmaa"ti ida.m
ekavacana.m, "aayasmantaa"ti ida.m dvivacana.m, "aayasmanto"ti ida.m
bahuvacananti? Tanna, yadi "aayasmantaa"ti ida.m vacana.m dvivacana.m
bhaveyya, "puriso purisaa"ti-aadiisu katara.m dvivacananti vadeyyaatha,
tasmaa buddhavacane dvivacana.m naama natthi. Teneva hi si yo a.m yo naa
hii ti-aadinaa ekavacanabahuvacanaa neva dassitaa niiti.

But why has no dual number (noun) been said in this noun exposition?
Because there is no dual number (noun) in the Buddha's Word, therefore
it has not been said.
But perhaps there is a triple number (noun)? For aayasmaa is a single
number (noun), aayasmantaa is a dual number (noun), and aayasmanto is a
multiple number (noun).
This isn't so. When the word aayasmantaa is a dual number (noun), then
which of the puriso, purisaa, etc, (in the above declension) would you
declare to be the dual number (noun)? Therefore there is no dual number
(noun) in the Buddha's Word. Thus neither the single number (noun) or
the multiple number (noun) is shown by whatever si, a.m, naa, hi, etc.

Nanu ca bho "su.nantu me aayasmantaa, ajja uposatho pannaraso.
Yadaayasmantaana.m pattakalla.m, maya.m a~n~nama~n~na.m
paarisuddhi-uposatha.m kareyyaamaa"ti paa.liya.m dve sandhaaya
"aayasmantaa"ti vutta.m, "uddi.t.thaa kho aayasmanto cattaaro
paaraajikaa dhammaa" ti-aadiisu pana paa.liisu bahavo sandhaaya
"aayasmanto"ti vutta.m. Na ca sakkaa vattu.m "yathaa tathaa vuttan"ti,
parivaasaadi-aarocanepi a.t.thakathaacariyehi
vi~n~naatasugataadhippaayehi "dvinna.m aarocentena `aayasmantaa
dhaarentuu'ti, ti.n.na.m aarocentena `aayasmanto dhaarentuu'ti
vattabban"ti vuttattaati?

But, venerable,  hasn't aayasmantaa been said in relation to two
(monks)in the text (of the Mahaavagga: ``Let the aayasmantaa,
listen to me, today is a 15th day Uposatha. (etc.)''?
(And) in the texts (Suttavibhanga and Paatimokkha) ``Recited,
aayasmanto, have been the four cases entailing expulsion'' aayasmanto
has been said in relation to many.
Isn't it possible to say: ``It has been said in that way'' in the
declaration of (Sa'nghaadisesa) probation, etc, by the teachers of the
commentaries who understood the intention of the Sugata ('s statement):
``By one declaring to two (monks) , `Let the aayasmantaa remember' is to
be said; by one declaring to three, `let the aayasmanto remember' ''?

Sacca.m vutta.m, ta.m pana vinayavohaaravasena vuttanti. Nanu vinayo
buddhavacana.m, kasmaa "buddhavacane dvivacana.m naama natthii"ti
vadathaati? Sacca.m vinayo buddhavacana.m, tathaapi vinayakammavasena
vuttattaa upalakkha.namatta.m, na
sabbasaadhaara.nabahuvacanapariyaapanna.m. Yadi hi "aayasmantaa"ti ida.m
dvivacana.m siyaa, tappayogaani pi kriyaapadaani dvivacanaaneva siyu.m,
tathaaruupaanipi kriyaapadaani na santi. Na hi akkharasamayakovido
jhaanalaabhiipi dibbacakkhunaa vassasatampi vassasahassampi
samavekkhanto buddhavacane ekampi kriyaapada.m dvivacananti passeyya,
eva.m kriyaapadesu dvivacanassaabhaavaanaamikapadesu dvivacana.m natthi.
Naamikapadesu tadabhaavaapi kriyaapadesu tadabhaavo veditabbo.
Sakka.tabhaasaaya.m dviisupi dvivacanaani santi, maagadhabhaasaaya.m
pana natthi.

It is true that it has been said, but that has been said on account of
the Vinaya idiom.
But isn't the Vinaya the Buddha's Word? So why do you say that there is
no dual number (noun) in the Buddha's Word?
It is true that the Vinaya is the Buddha's Word. Even so, because of
being said as a mere distinction on account of a disciplinary legal act
, it is not included in all general multiple number (nouns). For when
aayasmantaa would be a dual number (noun), action-nouns with that
application would also be dual number (nouns), but there are no such
kind of action-nouns. For not even one who is an attainer of jhaana
endowed with the divine eye and skilled in the science of reading and
writing would see one dual number (noun) in the Buddha's Word while
surveying (it) for a hundred years or a thousand years . Thus in
action-nouns there is no dual number in words relating to a noun with a
dual number nature. In words relating to nouns which are not of that
nature also the action nouns are to be understood as not of that nature.
In one's own language there are dual number (nouns) with regards two,
however there is not in the language of Magadha.