In Noldorin medial nasal + spirant became a double nasal, such as nth > nn, as in ON. thintha- > N. thinna-. In Sindarin, Tolkien modified the phonology so that the same was true of lth > ll, with OS. malthorn > mallorn being the prime example. See PE17/50 or VT42/27. Thus, S. bellas is perfectly appropriate.
However, when the spirant was final it was converted back to a voiceless stop, as in N. and S. thent. Ditto for lth, e.g. N. and S. malt, so belt is likely the proper Sindarin form.
+Paul Strack Yet Tolkien is quite insistent that such a cluster would be spelt in tengwar lambe/thúle, hence possibly belthas would be a better orthography. Compare glintha, antha and even malthorn in his post-40s documents.
Paul Strack Aug 07, 2015 (08:35)
However, when the spirant was final it was converted back to a voiceless stop, as in N. and S. thent. Ditto for lth, e.g. N. and S. malt, so belt is likely the proper Sindarin form.
Александр Запрягаев Aug 07, 2015 (10:57)