I have 4 V0838DZ engines in my db. Best guess, it was supposed to be V0831DZ, a Saturday. V0828DZ is known.

Rockers have 3 surfaces; just the vertical surfaces were painted black.

Style Trim codes are X11, X33. With the exception of the darker colors noted these cars would have black rocker panels. Black rockers were also part of SS equipment; X22, X55, X66 would also have them.

The Cortez Silver Z/28 in question is X77 and would not have black rockers.

The COPO notation is incorrect; had nothing to do with trim.

It appears they were never really released as they were initially noted "check availability." They are not mentioned on later revs. By January 2, 1969 they are listed with the optional equipment but are not on the standard paint chart.

Black, Champagne, and Butternut Yellow were initially listed as RPO colors in early 1969 Chevrolet marketing materials, then dropped. They built a few cars in those colors and they are not designated special paint. Later in the model year they returned as "special order" colors essentially meaning they were extra cost; $12.65 I believe. Later cars in those colors are also not designated special paint.

The car in question is a later build and likely - - PNT because of stripe delete; the cowl area does not appear to have been trimmed in white. It would make for a really rare, sharp Z/28 but it would be quite a project.

I believe the stripes should be black.

Spoilers were optional until early April '69. At that time they were included with Z/28 equipment and the option price increased to $506.60.

There is no way to know how many Z/28s were built without spoilers or striping.

Doors are the same as coupe, quarter glass is convert only.

It was a separate option, not included with or required with other equipment. 

OE Delco rear shocks had an integral stud for the lower mount. The double threaded bolt was common for aftermarket servivce replacement shocks.

The control arm was assembled to the shaft with bushings and hardware, dipped in paint. Ball joint [and bumper] was installed afterwards and remained natural finish. Which in WI is rust...

6500 red line tach, very rare.

No for both.

I work in Supply Chain. All you're probably seeing there is the difference between suppliers.

When a vehicle is in production and plan requires, say, 5,000 pc/week of some trim part it is not unsual to have several suppliers. That's a decent volume and a number of suppliers would be interested in a piece of the business. Sometimes you need more than one source as seasonal demand peaks may be more than one can handle. Some companies want a source within 1 day shipping from an assembly plant. So the source for rally wheel caps for Van Nuys may have been different than the source for Norwood. Cost is a very big deal to automakers. Even during production stuff is shopped around for a better price. Suppliers can be dropped for quality or delivery issues.

Service parts are a separate consideration. Service parts have to be individually packaged and high volume suppliers many not be set up to do that. So GMPD may designate one of the suppliers to send a number of caps per week to a packager. The cap supplied may not be an exact match for the original.

When a part is no longer current MY all bets are off. Volume drops off dramatically and the suppliers that were doing 5,000/week are often not interested. They had a line set up to do your part that is no longer justifiable. So now they have to set up and run a small quantity maybe once per month. Instead of a set up charge amortized over 250,000 parts the same costs will be incurred for 500. And it has to be packaged. Not unusual to see costs triple or more when they go non-current.

I have a much higher-resolution version of that photo (too large to post here), and it shows the unpainted backing plate quite clearly.

Firebirds used Pontiac-produced rear axles. Camaro rear axles were painted after assembly including brake drums and backing plates. Plenty of photos of this in vintage car mags.