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Since, as Mary says, you use "r" when it is a reprint, use "m" when
it has more than one applicable date.  "m" does not necessarily
mean a span of years, like in a set.  Although the dates and the date
code usually reflect the 260 $c field, they can reflect a variation
shown in the 260 compared with a 500.  LC also applies the use of
the "r" designation more liberally than would I.  They use it on
some materials which are only re-issues of the same thing, but
count it as a reprint.  I have seen this with books, but not
enough maps from them that any conclusion can be drawn.  But then
I learned cataloging from someone who took picune to an art form.
 
Debbie Lords
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