It's probably neither. At this time there was still a lot of flux in langur taxonomy (as there is now, but more so in earlier years). You'll notice that the sign gives full status to melalophos but retains the common name as applied to the formerly-lumped species.
P. femoralis changes colour from baby to adult, but this doesn't really look like one - and more importantly at the time I was only familiar with the bright orange form of P. melalophos so didn't think it was that species.
On balance it is probably a Sumatran melalophos, but it could also be something else entirely.
