Zenith Stromberg - ZS - SU -?

[MKCASPIAN1@aol.com]

Hello fellow listers - 
I know that Zenith Stomberg carbs are sometimes refered to as ZS carbs, but 
what do the letters SU stand for in SU carbs?  While were at it, what does 
the little M stand for on the bonnet latches?
John Flynn
78 Spitfire

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