71 demon master cylinder

So I went to the auto parts store and he checked
The numbers on the invoice and he said that this is
A master cylinder of a 71 demon with front disk brakes
:banghead: I always thought reservoir size was different
When using disks , so here is a pic of the new master cyl
Maybe I'm wrong but I don't want to plumb it and put fluid
in this thing and have it not work right !! So any help on this
Would be great. Thanks mike