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Maybe A. (cf.) sp. Nadelstreifen. They are found near Leticia where also a form of A. cf. bitaeniata is found. But without knowing the catch location or seeing better pictures of the fish in different moods, this is merely one of several possibilities.
"don't always" is quite euphemistic ;-) Show some pictures of them, preferably in different moods, then it should be possible to tell whether they are in fact D39.
Btw. the correct name is A. sp. D39.
Watch out for the metallic green/blue lip spot shown only by males of all agassizii-sublineage species. It is found on the lower part of the upper lip near the corner of the mouth:
The larger fish (male) shows a caudal pattern not typical of A. uaupesi. It could be A. lineata, but from this photo alone and without knowing the catch location, it's impossible to tell for sure. The smaller fish is for sure not uaupesi because it's a macmasteri-group species.
I see nothing that speaks against viejita - it might just be a particularly colorful specimen.
However, I have seen similar fish showing more red coloration on the head than typical viejita that were said to be from Cumaribo (Vichada). So, I would better call these A. cf. viejita and never keep...
Mostly gut feeling supported by the shape of the head and body and the narrow lateral band. If your fish show abdominal markings or horizontal caudal pattern or a downwards widened suborbital stripe as in the following picture, you can be 100% sure:
It's for sure a regani-group species, but the regani-group is quite large!
However, based on the distinct pale area between lateral band and caudal spot, it can likely be narrowed down to a regani-complex species. Unfortunately, I can't tell which one, especially if it's in fact from the Rio...
It was believed to be A. uaupesi before it's scientific description. A. uaupesi has several common names, see here. However, whether the above fish are in fact flabellicauda is impossible to tell without seeing the caudal pattern of the male and without knowing the catch location.
Females do show a very pronounced lateral spot when in spawning/brooding mood, but then it's easy to identify them as females anyway ;) Other than this, I'm not aware that females do show a lateral spot more often than males do.
What is the size of the first fish?