I would think yours is a 1930s or early 1940s? The cracked one I purchased was somewhat smaller then the ones Steve has on display in his ETF museum. Look at the two image dissectors at
http://aade.com/tubepedia/1collection/tubepedia.htm Yours looks similar to the image of his friends tube. Maybe my $400 estimate was a little low? There are a lot of WW-II sniper image dissectors advertized from time to time on eBay. I have picked up three of them over the years. They go from $10 - $150. A lot don't sell at $40. They are only about 5 inches long and an inch and a half in diameter. So I believe if you look at pictures on web sites discussing the properties of the images disectors you can find ones which match yours. I believe Farnsworth was always trying to improve the image dissector so the newer ones had features you won't find on the older ones. A 1933 patent of Farnsworth image dissector
http://patentimages.storage.googleap...S2087683-0.png