Ah a nod to Kenny Everett ! Remember it well.
I think what you have said is correct about the timeline, certainly designed as a 6 wheeler - hence the short tall fuel cell - but was it ever run on track as a 6 wheeler before we saw the 4 wheel version? I can't honestly remember.
If not then that would account for why the press designated it as the 'B', if they saw the 4 wheeler first that would be the 'A' version in their eyes, and the error would then go down in history (again). They would not have been aware that it was built up in the workshop as a 6 wheeler before they saw it run.
If my understanding is correct, 'conversion' to a 6 wheeler on the FW07 effectively involved bolting on the twin rear axle assembley on the back of the DFV and removing the conventional single axle. Then a different set of side pods was fitted.
I am pretty sure the differences between the twin axle, when bolted to FW07 front or FW08 would have been minimal, although there would have been some differences in the side pods to accomodate the different chassis width and length and cooling systems.
In that case, the conversion that you have described would be to fit the FW07 rear end. That probably explains why the first car appeared to have an FW07 rocker rear end fitted, when you might have expected something matching the pullrod front.
In effect there was no FW08 4 wheeler rear end, it ran with the 07's ?
If anyone who was there can correct that please do, i'm just going from observation of the different versions.
Incidentally the press made a similar error in trying to sort out the Lotus 87's and 88's, which i was able to do some research on, up here in darkest Naarfuk, but thats another story...