Alternate part numbers: 52K2171, 53K2171, CLS2511, TE505111 ( Moss Motors UK).

PART NO.  SPL.PAGE  DESCRIPTION                                                            
52K2171   15.A.4    Stud for tachometer pinion housing
53K2171   sup.7     Stud -housing (2) -(tach drive, engine unit, sup) 
53K2171   16.A.1    Stud -tachometer drive spindle housing (2) -(cyl.block, eng.unit, 1600)
53K2171   16.A.21   Stud -tachometer drive spindle housing (2) -(cyl.block, eng.unit, 1622)
CLS2511   TC.A.1    Stud for tachometer drive housing (2) -(cyl.block assy, engine, Twin
    Cam)
CLS2511   TC.A.9    Stud for distributor drive housing (2) -(case front timing, engine,
    Twin Cam)

High strength stud (55-ton).
Length = 1-3/8"
Top end thread = 5/16-24-UNF x 5/8"
Bottom end thread = 5/16-18-UNC x 15/32"
Notice that original CHSxxxx studs have short dog point on top end, which males it very easy to start the nut.
CLSxxxx studs may not have the dog point.
A leading "0" means plain stud. -- A leading "2" means zinc plated.

This threaded stud is used to attach the tachometer drive housing to engine block for all MGA (including Twin Cam). The numbers 52K2171 and 53K2171 appear in Service parts lists for 1500 and 1600 engines respectively. Not sure if one of these numbers may be a misprint, or if the part really did have two different but similar part numbers. It was common for the Twin Cam model to have new part numbers assigned to the special new engine pieces. Sometimes they were later reconciled and changed to a standard part number. In this case it didn't happen, so we have all of these part numbers assigned to essentially the same physical part.