What are SPU cartridges?

Searching for info on what these are as part of my educational process. All I understand it that it is an older design and perhaps it had a built in transformer in the cartridge. Looks like Ortofon still makes them. Is this a cartridge to just use on vintage gear? Does is have viability on modern tonearm designs?
If you type Ortofon SPU into Google you will have more material to read than the forums at Audiogon.

Short answer, it was a design from long ago, a cartridge that I myself owned in the early days of my stereo ventures.

Do I miss it? ....... NO.

Modern cartridges are much superior, including the ones from Ortofon, not to mention Lyra, Air Tight and Dynavector. Yes, the SPU is romantic, much like a Dynaco stereo 70.

Yes, it makes pretty music but it's out of date and in my opinion, the Denon 103 would probably be a more accurate reproducer of music and choosing the Denon would save you enough money to acquire something else nice for the system.

Others are welcome to disagree. That's what high end audio is all about :^).
I do agree, recently auditioned the TW Accustic Raven one, with a 12" ortofon arm and an SPU. Great deck, I bought it, but the cartridge was bright, hard, not for me at all