The other possibility with 7 inch front brake is that it is the similar type used on some unit singles. If the whole wheel is from a unit single then, as the offset is different due to different fork centre measurements, the rim will be off centre for your bike.
Photo is from C15 parts book and shows the setup used on some competition models, similar used on some B44 and C25. No nut to hold brake plate and spindle which runs in near side bearing with no sleeve.