I have used both 5L and 6L steel-bottles on my Dolphin.
I do use the converter. -Alternately, Martin @ tecme has replacement DIN-plugs for your Dolphin 1. stage
-Same amount of parts...
Now I plan to use two 6L-bottles, one on each side of the nice steel-chassis I have gotten my hands on... The trim is much nicer now...
But, plain old plastic-shell with 6L steel, has worked excellent for me.
Jon O