I believe the Auto Parts store is leading you down the wrong path here.
There, I Believe, is but one Disc Brake master for the 74 Maverick, a Non Booster type.
If they are stating two disc brake (Booster type/No Booster) masters, their book is incorrect. I don't think there was Power Disc in 74 Mavericks. The other might be a Drum/Drum master
But anyway the difference in a Power Disc Master, and a Non Power. The depth of the in the rear is definitely different.
The Manual , with push rod, will have a machined ring in the push rod hole at the very bottom of the hole. This accepts the clip ring on the push rod to lock in the push rod from falling out. ALL Manual masters have this. Power Disc Masters are smooth.
It is easy to seperate Power Disc Masters from Manual simply by looking for the machined area in the hole.
As far as Bore size, yes, Ford had MANY variations. If you choose a master anywhere from 7/8 to 15/16" all will function fine, just stay away from the 1", and over.
If you get a 74 Maverick, Manual Master it will come with the push rod----Don't clip it in for a 67 68 69 70 Mustang, it will be to long, making your pedal to high, use your old push rod to maintain the pedal heigth you are accustomed to.
On the 65 66 Mustang, go ahead and use the new Maverick push rod, your pedal will be Perfect heigth.
Classic Mustang Disc Brake Conversions and Power Steering