i see this on a few similar sabre hilts, noteably the one on this illustration appears to have that feature.
and this one:
just a reasoned guess:
the 90 degree corner there is a weak point & in a casting especially would require a radiused section on the inside as re-enforcement. the decorative lines may be just that, or as this area is also used to attach the sword knots (used as a lanyard to keep you from accidentally dropping your sword while on horseback, which could prove embarrassing in a battle) it could serve as a restraining device to keep the knot in place. many hilts of course have a slot near the pommel for this (like the heavy cav. sabre in the photo).