The valve in the picture really doesn't match up to what a "DISC BRAKE"
proportioning valve looks like, when you compare them.
If it's of help to you, here is a photo of what a "A" body disc brake prop valve looks like.
Install the disc brake one, when the time comes.
Are you sure everything off your donor car is original to it?
Meaning, all the disc brake components?
Looks to me, your donor car prop valve, is a regular drum brake one already.
