"Typical" Productivity Gain from SCM?

[CSI]

Hello,

I'm trying to find ballpark figures of "typical" productivity gains
from using plain-vanilla software configuration management (whether
it's centralized or distributed), like CVS, Subversion, Mercurial, or
git.

I've searched for literature that has hard figures, and am unable to
find anything that has that... I am a user of SCM for a couple of
decades, and am a firm believer in it, if only because it removes
chaos, but I would like to know if anyone has truly measured the
productivity gain.

If you can point me to any resources that have figures, even if they
refer to a single company or organization's experience, that would be
great.

I note that I have indeed found some numbers that describe the gains
of process improvement that includes many components, including SCM,
but that is not what I need: I need something that isolates the gains
of SCM by itself more or less, or at least as focused as possible.

Many thanks in advance, I've tried for a long time on this with no
luck.

C.S.i.