I think I was saying the same thing as you, but I was making the point that because the engine (whether 500, 650 or 400) can be an awkward fit on the mounts, that's probably why some people think that their 650 lump isn't the same as the engine they removed.
Once you have most of your engine bolts in the last mount somtimes won't line up until you apply a levering force with a strong broom handle, etc. I can see that if you don't know this it might appear that your new engine has slightly different castings.
And yes, all sorts of mid 80s Honda's had the same disk, that's why I have about 4 spare here that I picked up at autojumbles for bladders. GB disk is interchangeable with XBR disk, despite the absence of scallops to the inner edge.
Toodlepip
GC