I’ve been looking at that vent. I don’t think it will hurt to remove some of the material as the black rectangle covers the entire opening. I hope you follow up on how well it works.
On a side note: I was looking at the Hohner Performer 37 the spit valve is identical to this one. Another sign that these instruments or parts may be being made in a relatively few factories?
On a further side note: How did you get the photos posted? I thought that wasn’t possible on here?