Yes; I have concluded that is spot on. D instead of X.
I have been getting PCBs made
in production without any difficulty, in the UK and for the last 20+ years, in China, since I got Protel PCB in 1992. That only ever produces the stuff with the separate .apt file.
Nobody has said anything whatsoever...
ITEAD made prototypes for me 2 years ago, also without a word.
JLPCB have had difficulties with it recently but later found somebody who knew what to do with it.
Is it possible that X merely includes the entire aperture list at the start of each layer file? Apertures are like fonts; there is no harm in specifying the whole list for each layer's data, because only those referenced will get used. In that case, a converter should be dead easy to make. One could probably do it with a script. I could look at some sample files and try to work it out but I wonder if there is a converter out there? A google just turns up a post in Viewmate's forum saying that Viewmate can be used to do it.
Another way is to import the design into Protel PCB 99SE and do the gerbers out of that. 99SE opens the Protel 2.8 binary .pcb file apparently successfully and there is an option in the (convoluted) gerber output feature to generate "embedded apertures" which Viewmate displays fine.
Examining the two sets of files side by side doesn't reveal
identical data but then it probably would not, due to rounding errors in the floating point representation, etc.
Then I had an idea... I opened up PCB 2.8 and what do I find?