Vermeulen, O'Byrne, and Lamb; Bulbophyllum of Borneo; pg 73 - "... This species [Bulb. appendiculatum] differs [from Bulb. putidum] by the ridges on the adaxial side of the lip only reaching up to approx. 2/3 along the length of the lip, as well as in the distinctly convex part in front of the ridges." This results in the distal half of the lip appearing 'bulbous', see picture below from https://www.orchidspecies.com/mastappendiculatum.htm

Not mentioned but I have noticed that the basally upper margins of Bulb. putidum has a few 'hairs' (fimbriae??) along the upper margins whereas Bulb. appendiculatum upper margin is entire except distally.