I'll check up the standard stripe configuration for 1938 for you when I get home as I don't have it with me.

The first 20 or so HZ's were complianced 6/77 (I believe this is when the HX's rails were fitted with RTS upper control arm mounts). These are mainly brochure cars and other such internal stuff. Volume production started early 9/77 although there are also 8/77 HZ's at some assembly plants.

As Dr Terry says early HZ's have a fair bit of HX style stuff on them, which is normal. It is also normal for some stuff to change early, an example being the HX's upper control arm mounts. Sometimes a running change also occurs and people think it is a "HZ" or "HJ" or other such thing, but it is simply a change made for whatever reason (normally cost or reliability or commonality of parts) - example the full cable clutch setup people associated with WB actually was introduced into HZ in approx. alignment with Commodore. You find colour and trim varying across GMH's series' as well, eg HB Torana and LX Torana introduction saw changes to colour and/or trim in the current Holdens at the time (HK and HJ).
1974 AHJ complianced HJ's have a fair amount of HQ stuff on them too which didn't change until BHJ started.