Okay, what is this thing on my bail?

It's for power brakes and the first line is the last digits of the m/c part number.

"Mastershake" on Moparts wrote an article on correct master cylinder castings and part numbers, back when he worked for big brake company. He's made some updates and corrections since it originally came out but still a good starting point. I *think* the pdf version has the corrections.
Moparts - Members In Print, Brad Schroeder "Pumping Points", Mopar Action (jpgs)
http://www.moparts.org/Tech/MoparAction/Master_Cylinder.pdf

I started a cheat sheet here for A-body disk master cylinders.
It would be great if people add info to it.
Years___Application_____________p/n_______Bore
1966... Fixed Ft calipers, Manual.....................1"
1966... Fixed Ft calipers, Power......................1"
67-70.. Fixed Ft calipers.............2808600.....1"
71-72.. Fixed Ft calipers.............3461178.....1 1/32
73-74.. Floating caliper, Manual...3580146.....1 1/32
73-74.. Floating caliper, Power....3580112........15/16
1976?......................................____4647..........15/16

Some web resources for date codes at MMC Detroit:
Examples of date codes on parts

Understanding Date Codes