Joseph, the conversion cylinders offered by Fall Creek, Taylors, River Junction and others are safe, and they are all made by either R&D or Kirst, with most of the dealers being exclusive, as Taylors sells R&D, and River Junction sells Kirst.
I don't know if all guns chambered in 38LC can chamber the 38 Special, but the original cartridge used a heel type bullet at 375 diameter, so a proper chamber should accept that bullet, and if it will, the case diameter will be run out to the throat without a step, and would accept a 38 Special cartridge case. In the case of the conversion cylinders, they are short and will only accept a 38 special with a full wadcutter bullet seated flush.
No-one does and no-one will offer a 44 cylinder for the 51 or even the 60 Colt type. That is stated even on the Kirst website. The only conversion cylinders in 44 or 45 Caliber are for Remington type revolvers.