It seems strange to have covered 3"/50 mountings when similar destroyers had open mounts.
The Mk33 is the enclosed 3"/50 (open is the Mk27) and these were surplus USN in very good price (in erarly 1970's),so RHN bought about 40.On them a IR seeker installed in 1984,first IR system for guns developed entirely in HLK and fired new ammunition with proximity fuse or penetrating which improved their performance.Real Greek Navy has proximity fuse ammunitions for Oto Melara 76mm,giving some anti-missile capability.The Thyella is as decommissioned.