At last being an
old Eng has an advantage, there's nothing been changed or "Bastardized" there at all , many years ago this was the norm found on older sea strainers,
What you have is a large NUT holding the elbow in place some had seals other relied on metal to metal curvature ball face to seal, coming out of the strainer is a
MALE straight NST thread and yes 1 3/8 is A COMMON SIZE for NST Straight (Called NST) National Straight Thread),HAS 8 THREADS PER INCH, These came out in the same size 1 3/8 for 3/4 pipe and 1 " pipe (hose barb), they are designed with the nut so they can be adjusted to point different ways then locked up!
Hard to find the these days as have been replaced by NTS National Taper Thread , normally can be repaired by silver solder brazing if holed etc, cant find my pics as yet but below will give you the general idea of how it works/looks, this is plumbing fitting but you should get the idea!!!
Cheers Steve