I have a 1920's vintage tub faucet with shower riser. The faucet attaches directly to a clawfoot-type tub. The shower riser attaches to the top of the faucet. There is an adapter with nut that threads into the top of the faucet and the nut connects to the male end of the shower riser. The adapter is tightened into the faucet via an allen wrench.

I am looking for a new adapter, which seems to have CLOSE to a 18-20 thread. I have attempted to thread a standard 3/8" nipple into the faucet, but the nipple is slightly too large to thread into the existing faucet.

The adapter has a "shoulder" (or retaining ring) around the circumference of the body, which serves to capture the nut and is possibly a surface for an o-ring to seal to.

Can you help?

Forgot to add....there is no brand name on the faucet. However, "Patent # 1809567 2022875" is marked on the top of the diverter knob. I have attempted to research the patent #, but to no avail.

years ago we used to buy those faucets as part of a kit.
they were called pioneer shower for tub on legs.
i believe they came out of watertown mass.
i'm not finding anything on google.
the replacement faucet might be found at an older plumbing supply house.
i know the part you are looking for. i have never seen anything like that for sale.
P/S, i have torn out many of those and replaced with new.

It would be a proprietary thread. WHy do you need a different adapter?

Do you have... or can you provide pictures? Faucets from 1920's are no longer manufactured today because of the lead-free requirements. So many of the manufacturers that made those faucets are either no longer in business, or have been acquired by other business that stopped making the faucets and their unique accessories. Castings and custom machining is very expensive, but has been known to happen. The Queen Anne Showers by Milwaukee Brass, is a good example of what was once popular, but not now.
Can you provide pictures?
Best Wishes

Anything from the 20s became obsolete during the "Great Depression". Few plumbing manufacturers survived.

Please find links for pics attached. I need to replace the fitting because part of the retaining ring has broken. The ring acts as a shoulder for an o-ring, I believe, but also captures the compression nut (shown in the pic). Thanks all for your help!

i'll bet you could fashion a copper ring out of #10 wire. or cut the end off a stainless cotter pin.
that is if the head of that fitting is not too much out of round.

It may have been a Chicago Specialty faucet, but that doesn't help you because they merged with Dearborn Brass and probably dropped the faucet line.

I have the exact same number on my faucet. I found it to be a Moen. My question is whether I can keep the existing valve and replace the guts. The stems, seats, springs, etc. Are these available to fit???