The Seymour Duncan Iommi debuted in the mid-90s as an exclusive for Patrick Eggle Guitars' Tony Iommi model (both Tony and Patrick Eggle were from Birmingham). The guitar and the pickup were short-lived. When Gibson found out about it, they offered Tony a better deal for a guitar and pickup as long as he terminated the Patrick Eggle and Seymour Duncan deals. Which he did. (BTW, he was a complete gentleman about it).
One of the things that makes this pickup unique is that the blades are the actual magnets. So in this sense, it's quite a bit different from the Dimebucker and the rails pickups. Also, the bobbin that's used on this pickup is the same as the STK-S2 Hot Stack for Strat. That means it's quite a bit wider than a traditional humbucker, and it won't fit easily into a traditional mounting ring.
One of the first people to use this pickup after Mr. Iommi, was Mike Tempesta, who was Scott Ian's tech, and then went on to play guitar in Human Waste Project and Powerman 5000. (Nowadays, he's head of artist relations for Yamaha guitars).
There was a Duncan Designed pickup, the HB-104, that also had blades, but it was based on an HB-103 and (IIRC) it used a single ceramic magnet mounted traditionally and the blades were not magnets. It was used by a few Duncan Designed customers, including Schecter in the 5-string Mike Tempesta model.
Nowadays, you can find the pickup, re-dubed as the Scott Ian model, in Scott's new Washburn signature guitar.
And, you can get it from the custom shop as the Scott Ian Signature Model, blade-version. Also, we've tooled up a custom-made mounting ring that fits the wider bobbins.
I hope that helps.