It looks like the outer hub has been crimped, hammer forged? in some fashion not allowing the drum to slip over it, could just be illusion though but the fit is very tight at the hub. I tried WD40 and a BAH, usually works, but since I don't intend to use it anyway I didn't spend to much time on it.
It's two pieces no doubt, but how they are joined I don't know.

The original drums on this and the vert came apart as usual. Just this one oddball not coming apart.
I may save it for venting.

I think there's a bulletin in the FAQs that talks about pressed on drums for mid '68, '69 or something like that. If you drive the studs out first, you can more easily get the hub off. I learned this from Terry.

Here's a couple pics of my driver's side front.

Here's a shot straight on at it. I scraped the center hub part clean (was a hard, thin layer of grease under the black paint) to see if there was any signs that it may be intended to come apart. There is no gap whatsoever between the hub and drum, and no gaps between any of the studs and hub. I'm reasonably positive this was never intended to come apart.

As I recall back in the late '70's when working at Sohio , some front drums had to be ordered with the hub. I can't recall if this was a brake manufactur's desision or if it was spec.

https://firstgenfirebird.org/dtb/docs/68-I-69.pdf

Here's a good one from the archives.

Now that is some great info! And it shows you how the 'cards get shuffled' once in a while. That's exactly why some rules do not always apply. There are still some 'wild cards'.

So I guess when my car was built on the line, the DS installer got the notice while the PS side didn't

If I read that notice right, the one they show is the one we're calling the standard, two piece hub and drum?

I would hang onto that drum. Someone will need it. I bet that's why you have a mixed set. No replacement available so the previous owner had to convert one side.