Different books and efloras give different and contradicting keys, which are not actually correct on all accounts.
Book image in the Flora of Peninsular India gives, two sets of keys, out of which first set (based on calyx) may not be correct. Keys in 2nd set are below:
Leaflets ovate, acute, nearly glabrous, racemes very slender, near glabrous, pod glabrous- Teramnus labialis
Leaflets ovate- lanceolate, acute, strigosely hairy, racemes slender, villous with spreading hairs, pod villous- Teramnus mollis
I found the following entries in the “Bengal Plants”, on Teramnus species :-
(i) leaflets small, acute or sub-obtuse; racemes elongated, usually stalked. Leaflets membranous, sparsely hirsute; calyx teeth as long as tube —– T. labialis Spreng. (a slender twining species in all provinces).
(ii) leaflets small, acute or sub-obtuse; racemes elongated, usually stalked. Leaflets sub-coriaceous, densely pubescent beneath; calyx teeth shorter than tube —– T. debilis Prain (Syn. of Teramnus mollis Benth.) (a slender twining species in C. & E, Bengal).
(iii) leaflets large, acuminate or acute; racemes congested, sessile; calyx teeth shorter than tube —– T. flexilis Benth. (a rather robust twining species in Sundribuns and Chittagong).