50mB is a butane standard pressure that (was?) commonly used in Germany and I believe Scandinavian countries.
I have been trying unsuccesfully to source a new 50mB regulator for my (German bought) fridge for several years.
Peter Speedborough at CalorMarine knows of no source of 50mB regulators. He also says he believes the intention is to standardise on 30mB for butane across Europe (rather than the 28mB we use now, but close enough).