Hi Kostas,
As you are probably aware, The Stella's used what Nortronics called "B case" heads, a "standardized" shape that a few manufacturers made. They used Bogen (German) originally for PB and Record with Wolke erase, then switched to Nortronics PB/R.
I believe the only entity still making heads is AMR? or something like that in Europe - they made the later butterfly heads for the Stella SM. O yeah, I believe that Flux Magnetics also makes B case units - at least playback.
Depending on how many you need, I have some record and erase Pro Nortronics. Want to keep the few PB heads I have as there are no more. Also suggest contacting John French to see what he may have. I also have a few Pro nortronics heads that are fine but could use a relap.
Charles