Some errata and even more so PFNs imply minor changes of libraries
defined in SRFIs.

Can we maintain official version numbers (in the form of one to three
consecutive natural numbers) for these amendments?

The respective version number will be determined by the body
responsible for the erratum/PFN following some guidelines (e.g.
semantic versioning, starting with 1.0.0 for final drafts).

For Scheme systems supporting R6RS library versioning, these versions
can be directly used in the source code.  In any case, they can be
used for documentation in a particular implementation or a library
consumer.

Significant changes leaving the scope of the original specification
will have to happen in a new SRFI, of course.

The necessary work to maintain these version numbers seems minor to me
but will help portability, so I hope that what I propose can be
implemented.

Thanks for considering,

Marc