I have recently inherited a rather large box of various bike parts from the 1970s. I'm currently trying to sort through it all to see what's there and make some sort of an inventory. I'll probably use a small fraction and will eventually try to sell the rest.
So, the question is this: there are a couple of Campagnolo headsets and apart from simply trying the lockrings on some old forks (which I do not have lying around) I don't know how to tell what threads they have. Is it marked somewhere, and if so, where should I look and what should I look for?

It should be marked on the bottom of the locknut.

1 x 24 TPI is English/BSC/ISO
25 x 1.00 is French
25.4 x 24 F is Italian

It's easy to tell French threaded headset parts from British/Italian.

French/Metric is 25mm diameter and is too small to fit on a British/Italian steerer.

British/Italian is 25.4mm and will be loose on a French steerer.

British headset parts have 60° 1" x 24 tpi threads.

(British DOH) ITALIAN headset parts have 55° 25.4mm x 24 tpi threads.

While not a perfect fit because of the 55° vs. 60° thread angles they will interchange.

Switching aluminum alloy top adjustable cups and top nuts between British and Italian can wear down the threads.

French crown races almost always have 27mm IDs except Campagnolo used 26.4mm ID crown races for everything.

Some British and Japanese headset brands made both 27mm and 26.4mm ID crown races regardless of the steerer threads.

The head tube cups... if it fits, it work.

This is Italian standard, not British.

Thanks verktyg and T-Mar, that's very helpful.

DOH!

Sleepless night

Great info, but it might be that not every part is marked so it’s good to have a few forks loose that you can use for testing. And a dial or digital caliper to measure ID and OD.

Rather interesting that a two-year old post is still relevant in providing useful information. I have an old NOS Campy Sport (still wrapped and in the box) headset that I have never used because one time I tried to screw the locknut on to a Zullo fork and it just would not thread on. After reading the posts and seeing this picture I was able to identify the headset as Italian and not French which I thought the F meant! This time when it would not thread on by hand I took a wrench and after about one and one-half turns it threaded on, by hand, just fine.

"F." is abbreviation for the Italian word "filetto"="threads", and often caused confusion