I have an older Fenwick 320 with an aluminum ferrule. There's a slot for an O-ring (see attached picture). I'm curious if anyone still might still have stock on this part. Or, does anybody know the specs on the O-ring and where it would be available?

I've gotten them from a tray in Home Depot as well. Word to the wise- the size you need is a lot smaller than you may think. Get it tight.

O-rings are sized by both inside & outside diameter & the thickness of the rubber ring. The right diameter to fit in the groove may be too thick. Stretch out one smaller to make it thinner and it will degrade & break quicker.

The problem with hardware store O-rings is they are typically packaged and the store won't appreciate folks breaking open package after package checking the fit. In addition the drawers are usually a mess from folks doing exactly that so finding stuff and the selection usually suffer as a result.

If you have a dial caliper, you can measure the diameter of both the outside of male ferrule and the diameter of the groove to determine the approximate diameter & thickness required. Then look at O-Ring size charts which are out there on the Internet to figure out what may work. It really isn't that hard to dial in.

After that look on eBay. There are tons of O-ring sellers and you can buy a few sizes to try for what you would pay for one or two at a hardware store.

Good luck!!

Shrimpman gave good advice, Oring should be hard to put on, get smaller than you think.

"Size-matic." The O-ring was to work that trick in a ferrule material, aluminum, more dimensionally unstable than other metals. And to provide security against rattling or loosening while in use.
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I think you will be able to find one that works at any well-stocked hardware store. The thinner the better--bulging very slightly beyond the diameter of the male ferrule and soft enough to be able to compress very slightly smaller. Gosh, one of the older hardware stores near me has hundreds of them all jumbled up over the years in the little Servalite trays, priced 25 to 50 cents apiece. That's if you can figure out the stock number from the original section of the display, and then remember it all the way to the cash register. Otherwise, with this and any other small hardware such as machine screws, shims, sleeves, rubber plugs and so on, one dollar is close enough for me. Five and a half cents for the Governor if I have any change. Or go up the road to Ace where the boxes haven't been rummaged so many years, write the number down on one of the little plastic bags next to the tray, and pay the exact mount due, which could be twice as much.

As Carlz said, the ferrule may be fine without the O-ring. I have found and lost several over the years and just save the money for coffee and fish the same old beater rod without one.

If it ever gives me trouble like that, I will just replace the ferrule with a conventional metal ferrule. Probably I will find one of the O rings in the process.