This is a most attractive left hand dagger of 17th century form, obviously Spanish with the T stamp at forte which is as noted a compliance or acceptance type mark, I cannot recall what the OT means (memory slipping
. These were of course usually en suite with a rapier so the rapier associated must have been a beauty! Spain and Italy were the last holdouts using the left hand dagger in fencing, but by the end days most were simple quillons and ring guard except for the Spanish who held more to traditional styles.
The shellguard is distinctly Spanish as well, and these cockleshells were seen on many Spanish sword hilts. As far as I have believed, these have more to do with the military orders, primarily that of Santiago de la Espada, the Order of St. James whose symbol was a sword and the shell (la venera).
The inscription in the fuller is typically an acrostic for some invocation or patriotic slogan it seems.