In '71 they went with a sort of "one size fits all" disc brake master cylinder p/n 3461176 (B, C,E, and some light trucks). A bodies used an almost identical master cylinder (casting number and bore diameter same). The only exception I know of is the p/n 2944476 which was carried over from '70 for disc brake hemi cars.
The 2229171 casting number is the p/n 2944453 which is '70 e body disc brakes only. You can look on the fender side of the casting of your '9171 for a date code, the upper line will include "9175" which is some sort of assembly line or plant code, and the line below it will be the date code. "9 324" for example would be the 324th day of '69. Anyway, check that number and see if it seems at all close to late summer/early fall of '70 to get an idea of if there is any hint that one could be the original for your car. I've never seen a '171 on a '71 E body, so I doubt any were built with them.
But as they say with Mopars, anything was possible I guess!