Odd Vent Hood Duct Size (7.5"???) (Adding a microwave/vent hood)

[Spaceball1]

I'm in the middle of a kitchen remodel, and I'm getting ready to hang my new cabinets. Part of the remodel includes moving the stove over by about a foot, which means that the duct for my vent hood also needs to move over.

The duct exits out of the roof, so moving the outlet isn't an option. I'll need to install an offset (either in the attic or cabinet space, I haven't decided yet..) so I'm looking at all of the various fittings & such. Problem is, the existing round metal duct is 7.5" in diameter. I've measured 7" and 8" duct at the hardware stores, and both of those match their nominal sizes. The really weird thing is that the duct has no problem fitting on the collar for the hood where it exits the roof.

My home was built in 1979, so I'm wondering if 7.5" ductwork is an old size that's no longer in use.

Regardless, any ideas for adapting from the 7.5" size to a standard 6" or 8"?

[Spaceball1]

Well, a little more looking at it and I discovered that the existing vent was just bent. So it measured 7.5" in diameter left to right, but 6.75" or so front to back. So in the end, I tried it with some 7" fittings, and indeed it is a 7" duct.

Who ever said there's no such thing as a dumb question?