Sounds like you did OK on the AH Fox Sterlingworth. One of the many nice things about being a PGCA member, we have a section here for discussion of other doubles. I bought privately two Non-Parker doubles: A 20 Sterlingworth with 26" barrels, DT and extractors SN 261981 last November, and early this Month a 16 gauge M21 skeet with 26" barrels, SST and AE SN 20033- This very generous situation from our BOD saves us $, we can get information from brother Parkerites about our Non-Parkers and not have to pay the membership and search letter fees.
I know there is a AH Fox Assn., not sure about the Model 21, whether it is a entity unto itself, or is part of a greater Winchester Assn.--
How did you determine the year of mfg. of your Fox Sterlingworth? Mine has the Utica New York barrel markings, but the stock, grip, checkering, buttplate, outline engraving (graver stroll?)are identical to a good friend who has a 16 AH Fox Sterlingworth with the earlier Phila. Penn. barrel markings.
I am "spitballing here" but I don't believe Winchester made any numerical block distinctions between the 12, 16, and 20 gauge M21's from 1931 until 1960 at least. It is my understanding that AH Fox Co. assigned a block of numbers by both gauges, and also differentiating the Sterlingworth guns from the higher Grades with Alpha prefixes, whether ejector or not.
Another friend has a BE 12 bore AH Fox with Krupp barrels, original wood and metal finish and case colorings, and although it is not a Parker, it is indeed a finer gun. For some strange reason, the barrels are an even 29.5" dead on, but uncut vis a vis the ribs, solder and also the .729" NBD and the .025" choke right barrel and .037" choke in the left. It does have 2.625" shorter chambers, so he wisely shoots RST 2.5" shells in it.
My 20 has 2.75" chambers, I don't know the restrictions as to the chokes, but with 7/8 oz. No. 9 AA skeet loads it patterns about the same at 35 yards 30" circle as does the 16 M21 skeet with 1 oz. target loads.
To my way of thinking, choke "fussiness" and choke tubes are not the more critical aspects of a good shooting double- stock fit including pitch, barrel length and balance to the gun overall, and great trigger(s) and proper mechanics- safety, ejectors in time, etc-- are all more important--
By being "balanced to the gun" I mean this: a gentleman who requires a 15" LOP on his field gun, regardless of gauge (at least 12, 16 and 20 gauges) a set of 26" long barrels might seem out of proportion. And a M21 Duck Gun with 32" long barrels would seem equally out of proportion with a stock shortened to 13" for a Lady or a youth shooter-- IMO only!!