Full Idea
Dedekind plainly had fusions, not collections, in mind when he avoided the empty set and used the same symbol for membership and inclusion - two tell-tale signs of a mereological conception.
Gist of Idea
Dedekind originally thought more in terms of mereology than of sets
Source
report of Richard Dedekind (Nature and Meaning of Numbers [1888], 2-3) by Michael Potter - Set Theory and Its Philosophy 02.1
Book Reference
Potter,Michael: 'Set Theory and Its Philosophy' [OUP 2004], p.23
A Reaction
Potter suggests that mathematicians were torn between mereology and sets, and eventually opted whole-heartedly for sets. Maybe this is only because set theory was axiomatised by Zermelo some years before Lezniewski got to mereology.