Oxygen Regulator VIIIb

I see that in my collection I have a Mk VIIb oxygen regulator that looks very similar to a Mk VIII except that the supply level is covered with a blank disc and the wording above makes no mention of the level. Anyone know what type of machine this would have been used in?
I need a Mk VIII so should I convert this? or should I just mould a copy and make a dummy instrument.