Hi Curt, sorry for late reply. Yes, it is the ISO 25 stuff. There are other high contrast developers that will work, I just happened to be more familiar with D-19. Pyrocat HD is my main developer, but I could not get HP5 or Efke IR to print in carbon with it. Here is a scan of carbon transfer print made with Efke IR 8x10 and D-19.
Andrew, is the Efke IR the ISO 25 (?) speed. I have a couple hundred sheets in the refrigerator in 11x14. I've never used it, is D19 pretty much required for carbon printing? I was going to use Pyrocat HD. Does D19 produce usable carbon negatives? I've heard it suggested many times.