WINDSHIELD FRAME: Model A 28-29 and 30-31 interchanges?

I have a chance to buy a hot rod windshield (on eBay) labeled as a 28, 29, 30, 31 roadster windshield frame.

I'm wondering if this frame will work for me...

I have a 29 Phaeton front half (same cowl as Roadster or RPU as far as I know) that I intend to make into a bucket.

I see in the Snyder's Antique Auto catalog that the 28-29 and 30-31 have different part numbers.

What are the differences?
Is it a style or a fit difference?

Thanks!

--Matt

Good point. The seller listed it as a roadster windshield frame but said it fits his '29 Tudor, too. I may just have to let that one pass. Maybe I'll try to make my own from scratch. Has anyone done that?