It's been nearly 30 years so take this for what it's worth (read: my memory is *not* what it used to be), but:
This sounds like it might have been a US Doubler chip - the installation sounds very similar to the one I installed around 1986 or so in my drive.
What I don't know is if the US Doubler was a clone or licensed version of a kit sold under a different name, but I bought it from a UK retailer who advertised extensively in Atari User magazine. Happy Computers, possibly? My memory is definitely failing me on the name.