Mark, an 0 frame Breech face is 1/8" wider than a 00 frame.

I am pretty sure that the OP is looking at the .410 stock that is for sale on the internet and advertised as an 0. But it is more than likely an 00 frame. The seller should be asked for measurements of the head in order to confirm.
Since an 0 frame .410 would be a very odd duck, if not a non-existent duck.

Yes, All stocks are stamped with the grade number.
In the hammerless era at least.

1 5/8ths”

So O, OO, or OOO?

A few examples. Note the screw in 227044 used to hold the grip cap on grade 5 and higher guns.

Mark..... the only correct way to determine frame size is the exact distance between firing pin holes, center to center.

1-5/8” is off the charts. A 7-frame is 1-7/16”

But I think your measurement is the width of the stock head. But this is not the way to determine frame size.





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Thanks for the answer...and, especially, the pictures.

I have the stock that you are referring to. It measures a hair under 1 5/8 or a hair over 1 9/16 at the head.
Serial number 233299
I can't access jpg to add a photo at this time.

233299 is a VHE .410





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Classic Gunstocks has photos of a 20/28 DHE 2 barrel set on a 00 frame. They have photos on their Facebook page