I just bought a Fox Sterlingworth 12 gauge SxS double that bears serial # 859xx. Even though it has been "rode hard and put up wet", there is nothing like a true quality double.

It has the one line Philadelphia address and the "compressed steel" markings if that helps ID its' DOB.

Best,
Charles

Your Fox Sterlingworth was made in 1920. I'd have the chamber lengths checked before attempting to shoot it if that's your intent.
Jim

The chambers should be 2 5/8" in a Phildelphia mfg AH Fox 12ga if the gun is unaltered.
They adv that they cut them to 2 3/4 in 12ga early on (pre WW1) but seemed to purposely cut them 1/8" short,,not only in their 12ga, but also the 16 and 20.

The standrd lengths for the 16 and 20 of the day were 2 9/16 and 2 1/2 respectively. So these will measure out to 2 7/16 and 2 3/8 each.
You have to take into account the shell type and loads of the day as well as the abrupt forcing cone style of the bbl bore.

"Short shells' are most often used and advised for use in orig short chambered vintage guns in 16 and 20ga,
However Fox 12ga guns w/ 2 5/8" chambers are generally used w/ 2/3/4" shells and have been for decades after mfg. The Fox was sold as a 2 3/4" chmber after all.

Keep the vintage of the shotgun in mind when firing it (coming up on it's 100th B-day soon) and that most over the counter ammo these days is loaded to top SAAMI chamber PSI specs close to 11K to make sure the plastic stocked autos all work fine in the field. Not the type of ammo the Old Fox needs to do it's thing nor will be kind to stock bedding,,or your shoulder.

Many of these older guns have had the forcing cone of the bore extended to in effect relieve any preceived interference the old abrupt style cone and the purposely cut shorter than spec chamber may give to pressures. Most collectors and SxS purists gringe at the thought,,some shooters shrug their shoulders and pay the gunguy doing the work and feel safer about it.
There's lots of info back and forth about lengthening or not,, chambers and forcing cones on older guns. Do a search and prepare to be confused.

Here's link to a thread on the AH Fox collectors site that talks about the chamber lengths in the original Philadelphia guns. Lots of good infor over there about shooting and reloading for these,,that's all these folks fuss over.!
Fox Collectors • View topic - Chamber size of Sterlingworth 12 Gauge

They are strong, wonderfully put together guns. The Sterlingworth is the same mechanical gun inside as the Graded 'A-F' shotguns aside from the simpler forend release and forend lug needed.
Not a lot of problems with them.

Several companies make low pressure shells for vintage guns like yours. RST and Polywad are a couple of them. I have a Sterlingworth manufactured the same year.

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Why duck?? It's a 9mm!

I've had my 1923 vintage Sterlingworth for over 40 years. The chambers had been bored out for 2 3/4" shells before I bought it. Back in the 60's and 70's before pressure was really considered an issue with fluid steel guns this Sterlingworth absorbed everything for slugs to heavy duck loads without a hitch. I don't expose it to this type of use any more and the gun still locks up like the proverbial back vault door. IMO: The Fox's NOT the Parker's were the best built American shotguns of this period.
Jim

My only shotgun now, is a steelmaker from 27', I use 21/2" gamebore loads in it . Haven't had a problem