PART NO.  SPL.PAGE  DESCRIPTION                                                           
52K531    15.A.5    Stud for water outlet elbow (3) -(cyl.head, engine unit 1500)
53K531    sup.5     Stud -water outlet elbow (3) -(engine unit, sup)
53K531    16.A.5    Stud -water outlet elbow (3) -(cyl.head, engine unit 1600)
53K531    16.A.26    Stud -water outlet elbow (3) -(cyl.head, engine unit 1622)

53K531 - Stud -Stud for water outlet elbow (3) -(cyl.head1500, 1600, 1622, 1800)
Length: 1-1-7/8"
Top end thread: 5/16-24-UNF x 3/4"
Bottom end thread: 5/16-16-BSW x 15/32"

High Strength Stud

Alternate part numbers:
CHS2515 - BMC standardized stud pattern
328-605 - Moss Motors, USA
C3 - Scarborough Faire

52K531 and 53K531 appear to be identical parts, at least functionally the same. No idea why the part number changed between 1500 and 1500 Suppliment SPL's.

Notice: Studs procured from Moss Motors in summer of 2022 were soft steel (not properly hardened), and stripped the thread when tightening with normal torque.