'antara' appears to mean 'in-between', such as:
‘Mister, this is a bowl full to the brim with oil. You must carry it in between
this large crowd and the finest lady in the land.
‘ayaṃ te, ambho purisa, samatittiko telapatto antarena ca mahāsamajjaṃ antarena
ca janapadakalyāṇiyā pariharitabbo.
to regard 'anatara' to mean 'bardo' appears very imaginary (mannati/papanca)
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(in Tibetan Buddhism) a state of existence between death and rebirth, varying in length according to a person's conduct in life and manner of, or age at, death.
while i personally doubt the Buddha ever spoke this genre of sutta, my speculation is AN 4.131 refers to:
1. uddhaṃsotassa akaniṭṭhagāmino = non-returner with mind still stuck in the rupa & arupa jhana fetters thus akaniṭṭha upapatti
(continuing to delight in the Peerless/Akaniṭṭha brahma realm)
2. antarāparinibbāyissa = non-returner with mind not stuck in the rupa & arupa fetters but still with conceit & last two fetters thus bhava
keep in mind there is a sutta that says a stream-enterer cannot have an 8th bhava therefore the last 3 fetters naturally appear to be the types of bhavapaṭilābhiyāni AN 4.131 is referring to