This is an Isodynamic tweeter – more commonly referred to as a planar magnetic. The designer of this went on to develop units for B&G Radia.
This one has always been a favorite of ours. The tweeter is well constructed with a cast aluminum face plate. These are best used in 3 way systems or smaller two ways. Hi-Vi once featured a kit using this tweeter with the F5 woofer. It is quite efficient as well.
We’ve also made use of this device for repairing other, obsolete brands such as Panasonic, Foster,etc. We were able to remove the drive element and adhere to the Panasonic or Foster face plate. Legacy Audio used quite a few of the latter tweeters.