Need to remove vent grille, but what is this mounting behind?

[JimboMarimbo]

What is this raised border behind a bathroom vent/grille? The internal plastic grille just lifted off of four screw heads which are barely visible under the grime of ages. There is remaining a raised border which I want to remove, so that I have a flat plaster surface to work on: but what is this raised border and can I just chisel it off? Or is it part of an insert/sleeve going into the vent? I attach a photo of the dirt encrusted hole – you can see the raised rim of which I speak.

In order to fit a flat plywood panel with a circular 4” hole (to mount an extractor fan), I’ve removed the 30cm by 20cm plastic grille from bathroom wall interior, revealing a similar sized oblong hole to the outside where there is a external metal grille fitted.

This is a bathroom conversion dated to the early ‘70s, and I can’t see any fixing kits similar to my oblong grille i.e. with the four concealed screws for the plastic grille’s fish-holes to locate on.

If anyone can cast some light on the way this item fits together I would be very grateful.

 

[ree]

OP,
Why not post a pic showing the whole of that bathroom wall & a pic showing the wall on the outside?
If you are fitting an extractor in that bath room then the wooden trim around the hole can be prised off, and the hole bricked up. You will only need an extractor in a bathroom.
An appropriate sized hole saw could bore a hole in the most suitable place on the wall for the extractor to vent through. Presuming its an accessible first floor bath room then an appropriate terminal could be fitted on the outside. And the external air brick removed, & bricked to match.
But first the pics?

 

[Alastairreid]

It's an insert for an old metal grill.
It will lever out with a claw hammer.