They are TC Sounds 15" aluminum cone drivers optimized for dipole use. They were offered by TC Sounds at a very low price in 2003 to members of the Topica Dipoles list and the Bass List. They were strictly a closeout deal limited to stock on hand.
I plan to replace them with 8 Dayton RS225s per side which will allow experimenting with a higher crossover point. If I am happy with the result, I will shift the 15" woofers to IB use below 40-80 Hz. I am currently using an 18" Velodyne sub below 40 Hz.
The Topica list is no longer active but you can still read the posts. There are some interesting discussions about the RD-75. I noticed I had to login to read the messages so I am not sure if it is still possible to subscribe to this group if you are not already a member. Topica: Login