Before this gets any deeper (and I should have caught this when this thread was originally started), the Master Cylinder Casting Number that Richard is talking about, is for 1967 with Power Disc Brakes (Manual Drum and Power Drum Brakes for 1967 both used a Master Cylinder with Casting Number 2227061, with different internal parts).
One must also be careful in obtaining this Casting Number Master Cylinder (2227161), making sure that it came from a 1967-72 Mustang with Power Disc Brakes. I say this because this Casting was used in 4 different applications ; 1967-72 Mustangs, 1967 Ford Trucks, 1967-71 Chevy/GMC Trucks, and 1967-70 Dodges and Plymouths. All 4 applications each used a different Master Cylinder ; thus, not only could the internals of each be different, but also the mountings, size of the threaded ports for the lines, seats, etc.
I had a similar situation occur when I went to rebuild the 2227061 casting for the Power Drum Brake Booster I am adding to my '67. I had picked up a Master Cylinder with Casting Number 2227061 from one of our CMF members. I then found an original NOS C7ZZ-2004-A Rebuild Kit for 1967 Power Drum Brake Master Cylinders. I found that not only were the pistons different, but the ports (holes) in the seats were different. Luckily, for my 2227061 casting, only 2 applications are shown ; 1967-72 Ford, Merc and Mustangs with Manual Drum Brakes, and 1967-73 Ford, Merc and Mustangs with Power Drum Brakes. Nonetheless, I found that the ports on the Master Cylinder that I had obtained were 0.089" diameter, and the ones in the Kit were 0.116" diameter. While 0.027" inches difference doesn't seem like a lot, since pressure is inversely proportional to area, this can be a difference of more than double the amount of pressure that is put on the wheel cylinders. As I have heard many people saying before that they never replace the seats when rebuilding a Master Cylinder because they "look good", I thought that I should mention this. The one thing that I don't know about this though (since I noted that I purchased the Master Cylinder "loose") is did the casting I purchased come from a Manual Brake car, or did Ford make a change in the seats used in the Rebuild Kit ? Since my Master Cylinder for my Manual Drum Brakes had been replaced years ago with a Wagner Master Cylinder, I can't compare the Bendix one to it. Possibly someone has an original Bendix Manual Drum Brake Master Cylinder that hasn't been rebuilt, that they could check ?
Just thought that I should add this "word to the wise".
Bob