Whenever protection is added against a newly-discovered threat, the customer will receive a closing mail that includes a new Rapid Release sequence number. For example:
Determination: NewThreat Submission Detail: This file is detected as Infostealer.Limitail with our existing Rapid Release definition set. Protection is available in Rapid Release definitions with a sequence number of 174361 or greater. Signature Protection Name: Infostealer.Limitail Rapid Release Sequence Number: 174361
However, this number does not appear in the name of the IU file. The files are either given a static name (a name that always remains the same) or are given a unique name not intended for human consumption (like vd489e09.jdb)
To view the version, use 7zip or another archive tool to extract/open the .jdb file (used to deploy Rapid Release definitions to Symantec Endpoint Protection Managers, from which they will cascade throughout the environment) or symrapidreleasedefscore15-v5i64.exe (used to update individual SEP clients).
There is a catalog.dat file inside. (Look inside VIRSCAN.ZIP for symrapidreleasedefscore15-v5i64.exe files). Scroll to the very bottom for Version Information: