MGB rocker pedestal shims

[Bob Howard <mgbob@juno.com>]

   Some time ago there was a discussion about the shims (moss #460-255)
fitted under the center rocker pedestals of engines from 18GB on.   
    Can someone direct me to the archive for this, or tell what they are
for?  
    WIthin the last month our club members have taken apart six 18V
engines and have not found a single one of these shims.
Bob