Diddling a Tach

I've been chasing down some tach-indication problems lately, and as part
of it, I installed a borrowed tach in the Fly Baby. I'm ready to put
the original tach back in, but would like to change the hour meter to
reflect the time that was flown with the temporary tach in place. The
tach is a standard model made by AC.

Opening the tach is non-obvious. There's a set of screws in the back,
but there isn't a big enough opening in the back for the tach innards to
come out of. It looks like the faceplate of the tach needs to be
removed to let guts come out the face, but the faceplate looks like it's
formed around the mounting flange.

Anyone got a hint at how the tach comes apart...for that matter, will I
be able to just manually turn the hour-meter tumblers once the innards
are exposed?

I'm ready to rig up a power drill to turn the thing the old-fashioned
way, but since I'd flown about five hours on the borrowed tach, it'd
take about the same amount of time to run the new tach forward. Hoping
for a quicker solution.

Ron Wanttaja