Yeah it was something I had never noticed before as well. At least until someone brought it to my attention.

Here are the instructions that came with a old Perry Carb for a Picco 90 engine.
They show the slide valve and parts in the parts list and exploded view but they don't tell you what to do with it.
This carb was made by Varsane before Conley bought the rights to Perry carbs.




Of course the Standard Perry Carb for a K&B 61 engine, from circa 1987, doesn't show or mention the slide valve at all.