After the foundation of the MUTAVI Research and Advice group, the study of avian genetics moved rapidly within the group and new ideas and suggestions were proposed.
In several psittacine species two main fallow types have been recognized, a "pale" type and a "dark" type.
One of the proposed suggestions was that I personally was under the impression that "dark" red eyed fallows, showing a visible iris, might be allelic with the a-locus (NSL ino locus).
Recently we found first evidence that in at least two psittacine species this is true.

Some time ago I contacted mr. R. Nottet from The Netherlands. He is a prominent breeder of Cockatiels and was willing to try this testmating for MUTAVI in order to find out whether the NSL ino and the "dark" fallow are allelic in Cockatiels.
Unfortunately all eggs were unfurtilized in the first clutch, however, the second clutch brought 5 eggs, all fertilized. Non of the eggs hatched which was no surprize to me because it is a well known fact that NSL ino Cockatiels suffer from inborn weakness and are hard to breed.
Mr. Nottet pealed out all of the embryos and found that all had red eyes. I consider this as evidence that the NSL ino and the "bronze" fallow (dark type) are both alleles of the a-locus in this species.

Second evidence was recently found in Forpus coelestis by a prominent Belgian Forpus breeder. He mated a NSL ino to a "dark" fallow and all offspring displayed an intermediate plumage colour with red eyes.
Mr. Coppieters, another Forpus breeder, also stated that he already knew of the excistance of intermediate coloured fallows in this species producing NSL inos in their offspring.
I consider this as possible second evidence that the a-locus in psittacine species is able to develop a second allele known as the "dark" fallow type AKA "bronze" fallow. This phenomenon was already proved and confirmed in canaries for some years and also confirms my statement that fallows represent a (pigmented) type of albinism.

See for further reference:

In Quest of Events Leading to Pigmentary Disorders in Several Mutants of the Budgerigar.

Acknowledgement
I am very much indepted to mr. Nottet from The Netherlands and the (anonymous) Belgian breeder for their interest and cooperation in this investigation.