Thought someone here may know the answer to this: my mate recently picked up two HD's. One Melbourne, the other Adelaide built.

On the firewall numbers, the Melbourne car has another M after the first, the Adelaide car has an S after the A.

Anyone know what the second letter means?

Colin.

As Warren says, some cars had their bodies built, painted and hard trimmed at one plant, and were assembled at another. It is very common to see this in panel vans. Mostly these had their bodies built and hard trimmed at Elizabeth and Acacia Ridge (once it opended), and were assembled at Elizabeth, Acacia Ridge, Pagewood, Dandenong and Mosman Park (not sure i've ever seen a Mosman park van though). Normally you see Elizabeth bodies assembled at Dandenong, Pagewood and Mosman park in vans except for HQ where Pagewood did their own bodies. Vans are the most common ones to find like this, but there are others - i've seen a few HK GTS's with Pagewood-Elizabeth combos. For Elizabeth-Pagewood on HD and HR up until VIN plates were introduced you normally see the double letter like you have described with the Elizabeth chassis number, an Elizabeth BODY plate and the little tag from Pagewood. HK-12/69 HT and HR post 9/67 you get the same thing but a Pagewood VIN plate. 1/70 HT until the end of HG passenger vehicle production you also get an Elizabeth ADR plate. Once HQ rolls around you will only get the BODY plate from Elizabeth and the rest is all Pagewood (although this phenomena will be scarce in HQ as Pagewood did their own vans but only in HQ). I'm not sure what happened with HG utes and vans post HQ passenger vehicle production started, however I suspect these would have all been ELizabeth assembly, but maybe not. Elizabeth-Dandenong etc will follow the same system but replace Pagewood stuff with Dandenong.
There are also some very interesting early HK's that do not follow this trend. The bodies were built, painted and hard trimmed in Acacia Ridge and stamped with a Pagewood chassis number at the Acacia Ridge plant. They have a SS on the end of the chassis number (some I have seen have the second S sideways). The BODY tag has a Pagewood BODY number but it ends in S, and the model code is Acacia Ridge. So the BODY line looks like HK180M-00024S. All that have been found so far exist in a narrow PSN range. All perplexing stuff!