Field Background: I work for a medical device manufacturer.
We have several SOPs that state to reference the User Manual, however it does not state the revision of the User Manual to use. Is there guidance that state when referencing User Manuals or external documentation in SOPs that the revisions should be stated?
Nope, no guidance that I know of.
I think in such cases, most will NOT indicate a specific revision since that would require a spin of the SOP if the User Manual ever changed. Such dependencies are hard to manage / keep in sync. This really falls back on Document Control: does everyone know which version of a document is applicable at a given moment in time. However you do that in the context of your company is up to you.