This really is an interesting piece! and actually it is not a 'kastane' (the word derives from the Portuguese term for a decorative walking stick) but an 'interpretation' of one. The Sinhalese kastane seems to be primarily an 18th century form, though the earliest example of the well established hilt form appears to have been in the early 17th century. The ancestry for the motif on these hilts seem to derive from Hindu iconography figures, the figure on the pommel of course the Sinha (=lion) while on quillons are makaras.
Most examples of kastanes have the distinct hilt with quillon system that is believed to derive from those of Arabian nimcha type hilts, which in turn likely were influenced by early Venetian types.
This piece appears to have some sound evidence of age, and reminds me of the old chestnut; "a camel is actually a horse designed by a committee"
The hilt shows all of the correct features incorporated in the typical kastane hilts, however the arrangement is incongruent. The inner vestigial quillons on this interpretation are placed entirely below the upper quillon, and the extended rouded langet has nothing to do with the usual triangular type seen on normal kastanes.
Most interesting of all is the face incorporated into the knuckleguard, which does distinctly show European influence, in recalling the faces typically seen in the man in the moon face seen inscribed on European trade blades. These moon and sun faces are of course cabbalist symbols reflected on many 18th century blades with talismanic/quality implications.
The scabbards on this, as well as on the example Katana has posted seem to reflect the pierced and repousse work seen on sword mountings of the 18th to early 19th century in Hyderabad for export to Arab markets (see "Arms and Armour of Arabia" Elgood, which shows these, many with piercing and brass of 19th century), and the figure at the scabbard tip reminiscent of Ottoman scabbards as well as the thum on Omani janbiyya.
I'm unclear on the unusual blade shape on this, but seems to concur, at least in age estimation, from what I can see, probably 19th century. This seems to be something more intended for presentation or award, and while it would be difficult to place geographically by form, it seems to have been created in that period entering the complex trade networks from SE Asia, India and Arabia.
Extremely nice example, and though not a kastane variant, a very attractive piece. Its a great anomaly, just like I like 'em!!