Hi

THRD (Threaded)NPT (National Pipe Thread) UB (Union Bonnet) - SG (Solid Gate) - ISS.
ISS might be interpreted as Inside Screw Stem (Stem and Solid Gate/Wedge connected by means of thread) OR Internal Socket Seal (after the last thread of NPT, located some kind of O-Ring with function to act as primary Seal).
Better ask the originator who issue this inquiries to you. It would do no harm to ask since this abbreviation is not quite common, and usually used for SAP/database input

Regards,
MR

All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected

thank you Danlap