Those astute among you may have noticed that V3.5.4 was available earlier today, complete with a missing file 
V3.5.5 is the same version, with all files in tact.
If you downloaded V3.5.4 but do not plan to use the Customer Contacts part, there is no need to download V3.5.5
There is a database schema change, so you must back up your database first (but then you do that at least daily anyway, don't you 
)
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