When selecting South African CRS : HBK_NO_23 as the CRS for a new shapefile, QGIS creates slightlly different QPJ and PRJ files and creates a new user-specified CRS in the list of CRSes available in the QGIS desktop environment.

Steps followed to reproduce the issue:

Creating user CRSes¶

1. Start a new project in QGIS Desktop.
2. Create a new shapefile and select South African CRS : HBK_NO_23 as the CRS.
3. Look at the layer properties of the newly-created file - no CRS specified.:

4. Close QGIS without saving the project and relaunch QGIS.
5. Select Settings - Custom CRS from the menu; a new user-specified CRS has been added with a definition almost identical to South African CRS : HBK_NO_23. The only difference is that the user-specified CRS doesn’t have an +axis parameter set (values copied from QGIS Desktop Project Properties dialogue box):

South African CRS : HBK_NO_23:
+proj=tmerc +lat_0=0 +lon_0=23 +k=1 +x_0=0 +y_0=0 +axis=enu +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

User-specified CRS:
+proj=tmerc +lat_0=0 +lon_0=23 +k=1 +x_0=0 +y_0=0 +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

6. Load the previously created shapefile and look at its layer properties. Its CRS is now the user-specified one:

QPJ and PRJ files are not identical¶

The shapefile’s .prj and .qpj files have slightly different values for the datum’s degree unit (different number of decimal places and 295 (last three digits) in PRJ replaced with 3 in QPJ:

PRJ
PROJCS["Transverse_Mercator",GEOGCS["GCS_WGS_1984",DATUM["D_unknown",SPHEROID["WGS84",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",23],PARAMETER["scale_factor",1],PARAMETER["false_easting",0],PARAMETER["false_northing",0],UNIT["Meter",1]]

QPJ
PROJCS["unnamed",GEOGCS["WGS 84",DATUM["unknown",SPHEROID["WGS84",6378137,298.257223563],TOWGS84[0,0,0,0,0,0,0]],PRIMEM["Greenwich",0],UNIT["degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",23],PARAMETER["scale_factor",1],PARAMETER["false_easting",0],PARAMETER["false_northing",0],UNIT["Meter",1]]

I don't think a user-specified CRS should be created when selecting a built-in CRS.

Also, I'm not sure if the differences in the QPJ and PRJ files might create problems when using other GIS software.