Packaging when upstream source filename doesn't change for revisions

[Eric Smith]

I'm packaging asl (a GPL'd cross-assembler package for many microprocessors,
by Alfred Arnold):

    http://john.ccac.rwth-aachen.de:8000/as/

I started from a spec file from OpenSUSE (also GPL'd), removed some
openSUSE specific stuff, fixed the builddepends, cleaned up the patches,
and updated to the latest upstream beta release.  It builds fine, and
I want to submit it for Fedora inclusion.  I've read the packaging
guidelines, naming guidelines, and Package Maintainer instructions,
but I have an issue that isn't covered.

The upstream maintainer does not change the source filename (or URL)
when he issues new beta releases.  They are always named
"asl-current.tar.gz".  Inside the tarball is a changelog that gives a
build number for the release; for instance, the latest ones are
prereleases of 1.42, and the changelog entry for the beta release
calls it Bld55.

Bssed on the naming guidelines, I think the SRPM should be
asl-1.42-0.x.bld55.src.rpm.  But my question is what to do about
the source file.  Do I leave it as asl-current.tar.gz, or do I
rename it locally as asl-current-1.42-bld55.tar.gz?

I haven't found any definitive guideline for this situation, and can
see arguments both ways.  If I leave the filename alone, it is not
possible to have multiple versions in the RPM build tree, but
perhaps that's not important.

Is there any official policy or guideline for this situation?

Thanks!
Eric