In 1970, there were 1,643 fastback Mustangs with the M code (351 cleveland 4v) and 3 speed MANUAL trans. Don't know the breakdown of how many of those were mach 1's. You could pay Marti for that info if you wanted it.
Keep in mind though that just because an option is rare does not make it valuable. It is how desirable the option is what makes it valuable.
What other info are you looking for?