I've now seen several of these at car boot sales etc., but still have no idea what they were for. My best guess is that it was to send the video output from the computer to some sort of signal converter or splitter box. Anyone know?
later - mystery solved. Found an eBay listing that included the product code 603-8471, it turns out to be for use with Apple's two-port DVI graphics card - their standard DVI plug was a little bit too wide for two to be plugged in side by side, so you used this adapter, whose main feature was a relatively small casing on the male plug.
Also posted at https://ffutures.dreamwidth.org/2213361.html, where there are comments. Please comment here or there using OpenID.