Your post got me thinking, so I found an old advertisement for the Tornek-Rayville.
In addition to mentioning 400 feet and self-winding, etc, it also mentions it's non-magnetic qualities alongside the symbol I mentioned. The symbol for magnetic permeability is a lowercase Mu . Which is what this symbol most closely resembles.
The special military issue non-magnetic Aqua-Lung twin hose regulator also has it, but it is more stylised, more of a 'w' than a 'mu'.
So now my hypothesis is that it marks equipment that is non-magnetic and suitable for certain operations (eg Mine-clearing).
This message has been edited by BDLJ on 2009-12-03 15:02:15