Getting ventilation grille off a wall

[Coaster]

Hell all,
can anyone tell how the grille is fixed to the wall.

Fitted about 20 years ago (not by me) the internal grill fitted to an external wall doesn't appear to have screws fixing it. as I recall, theres also no sleeve running to the airbrick outside.
it appears to be plastic but no obvious way of how its fitted. I want to get it off without damaging it if possible.

its squareabout 6 x 3 inches in old money dimensions.. louvered type but isn't adjustable.
it was fitted by the same guys who fitted the central heating at the time.

can't get a picture of it as I don't have any way of getting a pic of it.

anyone know if there were ones that slid onto position over screws or something like that.?

 

[ree]

If plastic, then its either a snap-on or, as you suggested, a slotted-on to four proud screws.
Cautiously try moving it left and right or slipping a spatula below it to feel for a sub-frame.

If its plaster then its been stuck on and wont come off without smashing.

 

[Coaster]

thanks ree.. will give it a go.

 

[kazuya]

it might be a plaster one, in which case it could be stuck on using finish or something, if so work your way around the edge with a thin instrument like a wall paper scrapper, then hopefully with a bit of gentle prising it will pop off.