I agree, your stamp is a 599.
Your stamp looks like it was soaked from a business envelope which received a private cancel applied by the mailer. These are typically the product of the Multipost Company of Rochester, NY and are written up in a monograph published in 1997 by the Machine Cancel Society titled "The Multipost Company: History, Machines, and Markings". Here are several examples showing the variety of killers which often appear on precanceled stamps like yours. It is very likely the Spring Valley, NY cancel was made the user.