The generally accepted pedigree of the Nicholas Montgomerys of Cubley, Derbyshire, which we have on WikiTree is:
I. Sir Nicholas Montgomery MP, 1356-1424 (https://www.wikitree.com/wiki/Montgomery-534)
II. Sir Nicholas Montgomery MP, 1376-1435 (https://www.wikitree.com/wiki/Montgomery-3958)
III. Nicholas Montgomery, 1404-1465 (https://www.wikitree.com/wiki/Montgomery-56)
IV. Sir Nicholas Montgomery, 1449-1494 (https://www.wikitree.com/wiki/Montgomery-52)
The lineage is attested by a pedigree submitted in a court case in 1515 by the last Sir Nicholas's son Ralph Montgomery (https://www.wikitree.com/wiki/Montgomery-211). See also Rosie Bevan's account on Soc. Gen. Med. (https://groups.google.com/g/soc.genealogy.medieval/c/gugpoXlN9nM/m/4m0IuDrG-UwJ).
Nicholas Montgomery junior signed a bond in 1452, which implies both that he was of full age and that his father was still alive. In 1462 Nicholas Montgomery junior signed a deed relating to property he was due to inherit from his deceased mother Eleanor on the death of his father Nicholas. However, in 1465 the heir of Nicholas III was another Nicholas who was a minor. This latter Nicholas (IV) was of age in 1472, when his mother was stated to be Isabella (née Vernon, widow of Nicholas Montgomery, then married to Nicholas Agard).
The above leads me to speculate that there was another Nicholas Montgomery missing from the accepted genealogy and from WikiTree. This hypothetical Nicholas would have been born between 1425 and 1430, the son of Nicholas III and Eleanor "of Cheresey"; and may have been the father of Nicholas IV (who would thus have been the grandson, not son, of Nicholas III). He must have died between 1462 and 1465, before his father, and therefore did not inherit the Cubley estates and must have been omitted from his grandson's pedigree.
This is not a new hypothesis: see for example https://soc.genealogy.medieval.narkive.com/GL5omaxQ/maud-furnival-wife-of-walter-montgomery-of-cubley and https://www.genealogieonline.nl/en/genealogie-richard-remme/I286461.php .
I am trying to obtain copies of original documents referred to in the secondary sources (including Nicholas III's IPM and the 1472 law suit). Any comments in the meantime?