I have no idea what manual/manual means. And just because a counter guy says it fits both applications (which it will), it does not always mean a part is "designed" for both applications. There is a difference between a 72 C10 power brake system master cylinder and a non-power brake system master cylinder - and some parts houses carry separate part numbers and list applicability based on power vs. non-power. Believe bore dia is different as mentioned above - and also that the depth of the depression in the back of the piston varies between power (shallow) and non-power (deep).
Here is an LMC example: