Well fellas - as they say, a picture is worth a thousand words:

SXP Replacement barrel for 3.5" compatible receivers:


SXP Replacement barrel for 3" compatible receivers:


Looking at how each barrel type engages the receiver, it looks pretty clear the receivers, and the barrels, are different. Interesting to see that, and very odd, but it does indeed look like the receivers and their respective barrels are different.

Both pics stolen from cheaperthandirt.com's item pages, but if you dig around Winchesters site on this particular mission (ain't easy to find the pics), you'll find the same difference - specifically I found the pics with the same barrel tennons on these pages: the buck/bird combo model is a 3" model and shows a barrel tennon which matches the 3" barrel above, and the barrels page under products has a 3 1/2" barrel depicted at the top of the page which matches the 3 1/2" barrel above.