Not sure if this any help, but would like to point out that the 1970 MTSC has 'replacement' master cylinder numbers.
http://www.imperialclub.org/Repair/Lit/Master/274/page13.htm

and going back further, the 67-3 shows a flat front disk brake master.
http://www.imperialclub.org/Repair/Lit/Master/232/Page10.htm

Granted appearance in service manual of MTSC is not proof of actual use for particular, make, model year...

Also, the magazine articles reprinted by Brookland show both '67 and '69 Barracudas with bail type flat front MC. Can we deduce anything from that in terms of timeline?