I found a some extra pieces of some O72 curved pieces stashed away in a box.
Early production, solid rail stuff: No markings to indicate the curve size.
Later production, hollow rails: "O-72" molded in to the underneath side.
The chord length (straight-line measure) of the center rails measures right about 14-1/32". However, I also noticed some O54 curved track on my layout measures almost identical, right about 14" too. But this is due to the O54 curved track being 30 degree curved segments, while the O72 curves are 22-1/2 degree segments.
I would imagine that O82 curved track is the same way. I have a four curved pieces of O82 later production hollow rail track on my layout acting as easements, but unfortunately, they are screwed down, and I would need to unscrew a handful of track sections to remove and get a peek at the underside to confirm that they are marked. With the chord length of the center rail being real close to 16" however, it should be fairly hard to confuse these with any other MTH curved pieces.
Hope this helps.