I've never seen a non ribbed bbl for the 20 ga 870 not on a shotgun.
The "Youth" models have interchangeable parts within gauge with the standard models. There was a "Special Field" model 870 (and 1100) some time ago that had a straight gripped stock,there were a few parts that did not swap with others.
Most "Yute" models are 20 gauge, including the 870 YE here I got for the kids. Relatively little aftermarket stuff is made for the 20.
Alla same-same, a stock YE 870 is an excellent HD shotgun, decent upland gun and can be used for deer hunting with slugs. Very versatile, just not lots of extras around.
HTH...