Screamingmorris,
You might also check on the valve stem. There should be a line scribed around the stem just at the place where the stem disappears into the valve cap. Those lines should be lined up with the top of the cap. It is supposed to be a quick way to see if you have the proper alignment.
I found this on the Conn Loyalist site with this article being the one with vertical alignment info (
The Conn Loyalist) This works on my '47 22B, and I would guess that it would apply to your '52.
The old Conn mouthpieces seem to work best on these great horns. I have had a real devil of a time trying to mate up a mouthpiece to the horn, and lo and behold, the Conn stuff works best! Go figure.
Good luck,
Bill