Circa: 1921 - 1934
Zeiss/Ikon was formed in the mid-1920s by the merger of five major German companies: Carl Zeiss, A.P. Goerz, Ernemann, ICA and Contessa-Nettel. Some items (like Contessa cameras and Goerz optics) held their brands. Others were consolidated into the Zeiss/Ikon name.
This extinction meter was developed by ICA before the merger; so pre-merger Diaphots bear the ICA brand, and post-merger units are Zeiss/Ikons. Exactly when the changeover occurred is unknown; very likely they took a few years to retool and sell the existing inventory. Mine is a Zeiss, so it was made sometime between 1925 and 1934.
Beautiful little extinction meter. It's largish in diameter, about 3-inches, but extremely slim, so it'll slip into a pocket with almost no bulge. The front is very shiny and it's got that almost transluscent star on the front, and the chrome brightwork along the edge. Mine came in a nice hide case that fits it like a glove and adds barely any thickness to it.
On the back there's a little window and inside there's a graduated blue filter. You aim it at your subject and turn the dial until the image disappears (or as good as you can get), then flip it over and read the combinations on the front. Like all extinction meters, it favors older people whose eyes don't adjust as quickly to different light levels. Not terribly accurate, but better than Sunny 16 in iffy conditions, and it doesn't need batteries.
For more info, there's a very nice web page about it here.