It is a 6cyl HQ Monaro, optioned with 202HC, trimatic and metallic paint. Probably has front discs too. Assume column shift?

1845 is Patina Gold metallic as you state.
21E is Antique Saddle/Brown vinyl on a HQ Kingswood or HQ Monaro.

It should have a number stamped into the firewall under the LH washer spray pipe, something like 604xxS

All the numbers go together OK as far as I can tell. 60406S will be the firewall number. Not sure what you are worried about, 12/73 is a 1973 HQ. Any HQ complianced after 1st September 1972 is a 1973 HQ.

Forgot to mention , G95 under rear axle is 3.36:1 banjo. This is the standard rear axle when auto optioned on an 0337 HQ, but isn't the standard rear axle for an 0337 which is why it has a code. If it was a standard 0337 it'd be blank in all spots as it'd be a 173, 3spd column with 3.55:1 banjo rear axle.

I saw some more of your questions to:

H754805 is the Production Serial Number. You sometimes find info that breaks this up, even GMH did it themselves but that is simply silly and confusing. This number is the pivot column by which most surviving GMH records are listed so it is best left as it is. If you try to break it up like you have into a plant code and then a serial number you end up with the same serial number being used every 100000 PSN's issued.

On these cars nothing matches anything, other than the chassis number being stamped into the Safety Compliance Plate (I call it the ADR plate to make it simpler). But with enough cars recorded it is easy to see a correlation between chassis #, PSN, body number and firewall number (for Pagewood bodied cars like yours), and hence spot a car where someone has swapped tags between bodies. Yours all fits together OK given others known around it. You can see that the following numbers are close:

60406S firewall tracking number. This is the number the Pagewood plant used to track the car.
60374-S body number. This is the 60374th body to be affixed with a body tag in the Pagewood body plant. By the number you see things get out of whack slightly with the firewall number.
CHQ60438S, 60438th chassis number on a HQ at Pagewood. Pagewood chassis numbers went on in the assembly plant hence they are also screwed up relative to the other two, but still close. As can be seen others overtook this one.

Edited by user Tuesday, 1 December 2015 4:36:19 PM(UTC)  | Reason: Added more info

....the firewall tracking number may relate to the bare shell "skeleton", I'm pretty sure to potentially see if any production issues (cracks etc.) occur around a certain body shell sequence or "block", whereas the body number is actually the "body assembly number" which comprises all attached doors, boot glass and interior trim (istbc). The body assembly at that point becomes subject to warranty in house. In the corporate GM scheme of things, the GMH body plant actually "sells" the body assembly to the car production plants....hence having to warranty the body assembly.


....This applied to Holden production in 1948, and I'm pretty sure the same numbering procedure applied through the years....Cheers

Thanks to all especially HK1837, I will be a proud owner of a HQ Monaro and I will be gathering as much information about the car as I can to keep with it.