Most cockpit drains are simply thru-hulls fittings with a barbed end. It sounds like you have one designed for a seacock, hence the NPSF thread.
The only company that makes an NPS female threaded tailpiece is Forespar. You might try Hamilton Marine, Defender Marine or West Marine. I know Defender has them.
Couldn't find any other manufacturers making NPSF threads.
The Sabre MK 1 is not a new boat, if the cockpit drains are old and plastic it might be cheaper in the long run to replace the drains with the threaded to barb versions. You'll have new drains, well cocked, and fewer fittings.
Fair winds