you are correct on which number is the date code, but not on deciphering it. 7 040 is the 40th day of a year ending in 7, most likely '77.
Bendix made these master cylinders for at least 6 or 7 years after the model year as replacement parts, a master cylinder is a wear part.
Look closer at that 03490 number on the bottom. I bet the two zeros are actually the imprints of phillips head screws that the foundary used to screw a date code plate into the mold pattern. That would make the casting date 349. Casting casted 49th week of '73 probably.
I'm lost on your last question, inny, outy, are we talking about belly buttons or your E body master cylinder?