Hi Hendrik, Nissan sold multiple kits:
* Repair Kit
* Screw & Washer Set A
* Screw & Washer Set B
* Gasket Kit
some Keyster brand kits combined all into one
Only the GASKET SET is normally needed. However, if your carb leaks fuel into the fuel bowl (overflows) you will need new Valve assemblies. For A12GX, it uses the same valve assembly as the DCG306 downdraft carburetor. Or you could get the Repair Kit which contains Valve and Needle Jet. Make that two repair kits as each kit only contains the parts for one carburettor
What's mainly different between the Kits are the needles which differ between early GX, later GX and 510 1600/610 1800 and Fairlady 1600 kits
HJE38W was for the 1970-1972 GX carburetors. In 1973 they changed to the new style (with IMS idle system) in the Japan market. I don't know what what used in Europe or South Africa. The IMS system uses different gaskets and has flat-top chambers. Some flat-tops don't use IMS
For photos & listigs, see
A12GX Carburetor Part Numbers