Andy I really don’t know what your issue or point is - I don’t doubt that these two species are very similar and call is a good separating criteria of birds not seen clearly. However, there are morphological differences that have been cited in more than one reference so it’s disingenuous to imply I am suggesting this based on one source. Larry cited a source above too which notes
“The Mountain Serpent-eagle is distinguished from the Crested Serpent-eagle by having longer wings, a darker plumage with black throat and broader white band on the tail. When seen flying from a long distance, it is probably easier to id from the voice, with the final note of the Mountain being more sustained.”
also at close range
https://horukuru.blogspot.com/2015/07/mountain-serpent-eagle.html in addition to the previous observer believing the bird is ‘stockier’ than a CSE so not contradicting other observations..
and here
http://animal.memozee.com/view.php?tid=2&did=41100
However, as like with every other person who has commented, I have never seen a Mountain Serpent Eagle, so like you can only go what I find on the internet and from personal experience of iding Crested Serpent Eagles in other locations.
I’ve not really anything else to add - the original ID is correct so that’s all that matters.